Small improvement

app/src/main/java/eu/faircode/email/AdapterAccount.java

@@ -505,7 +505,7 @@ public class AdapterAccount extends RecyclerView.Adapter<AdapterAccount.ViewHold
         diff.dispatchUpdatesTo(this);
     }
 
-    private class DiffCallback extends DiffUtil.Callback {
+    private static class DiffCallback extends DiffUtil.Callback {
         private List<TupleAccountEx> prev = new ArrayList<>();
         private List<TupleAccountEx> next = new ArrayList<>();
 

app/src/main/java/eu/faircode/email/AdapterAnswer.java

@@ -267,7 +267,7 @@ public class AdapterAnswer extends RecyclerView.Adapter<AdapterAnswer.ViewHolder
         diff.dispatchUpdatesTo(this);
     }
 
-    private class DiffCallback extends DiffUtil.Callback {
+    private static class DiffCallback extends DiffUtil.Callback {
         private List<EntityAnswer> prev = new ArrayList<>();
         private List<EntityAnswer> next = new ArrayList<>();
 

app/src/main/java/eu/faircode/email/AdapterAttachment.java

@@ -383,7 +383,7 @@ public class AdapterAttachment extends RecyclerView.Adapter<AdapterAttachment.Vi
         diff.dispatchUpdatesTo(this);
     }
 
-    private class DiffCallback extends DiffUtil.Callback {
+    private static class DiffCallback extends DiffUtil.Callback {
         private List<EntityAttachment> prev = new ArrayList<>();
         private List<EntityAttachment> next = new ArrayList<>();
 

app/src/main/java/eu/faircode/email/AdapterCertificate.java

@@ -195,7 +195,7 @@ public class AdapterCertificate extends RecyclerView.Adapter<AdapterCertificate.
         diff.dispatchUpdatesTo(this);
     }
 
-    private class DiffCallback extends DiffUtil.Callback {
+    private static class DiffCallback extends DiffUtil.Callback {
         private List<EntityCertificate> prev = new ArrayList<>();
         private List<EntityCertificate> next = new ArrayList<>();
 

app/src/main/java/eu/faircode/email/AdapterContact.java

@@ -414,7 +414,7 @@ public class AdapterContact extends RecyclerView.Adapter<AdapterContact.ViewHold
         set(all);
     }
 
-    private class DiffCallback extends DiffUtil.Callback {
+    private static class DiffCallback extends DiffUtil.Callback {
         private List<TupleContactEx> prev = new ArrayList<>();
         private List<TupleContactEx> next = new ArrayList<>();
 

app/src/main/java/eu/faircode/email/AdapterFolder.java

@@ -1132,7 +1132,7 @@ public class AdapterFolder extends RecyclerView.Adapter<AdapterFolder.ViewHolder
         return result;
     }
 
-    private class DiffCallback extends DiffUtil.Callback {
+    private static class DiffCallback extends DiffUtil.Callback {
         private List<TupleFolderEx> prev = new ArrayList<>();
         private List<TupleFolderEx> next = new ArrayList<>();
 

app/src/main/java/eu/faircode/email/AdapterIdentity.java

@@ -415,7 +415,7 @@ public class AdapterIdentity extends RecyclerView.Adapter<AdapterIdentity.ViewHo
         diff.dispatchUpdatesTo(this);
     }
 
-    private class DiffCallback extends DiffUtil.Callback {
+    private static class DiffCallback extends DiffUtil.Callback {
         private List<TupleIdentityEx> prev = new ArrayList<>();
         private List<TupleIdentityEx> next = new ArrayList<>();
 

app/src/main/java/eu/faircode/email/AdapterImage.java

@@ -223,7 +223,7 @@ public class AdapterImage extends RecyclerView.Adapter<AdapterImage.ViewHolder>
         diff.dispatchUpdatesTo(this);
     }
 
-    private class DiffCallback extends DiffUtil.Callback {
+    private static class DiffCallback extends DiffUtil.Callback {
         private List<EntityAttachment> prev = new ArrayList<>();
         private List<EntityAttachment> next = new ArrayList<>();
 

app/src/main/java/eu/faircode/email/AdapterKeyword.java

@@ -218,7 +218,7 @@ public class AdapterKeyword extends RecyclerView.Adapter<AdapterKeyword.ViewHold
     }
 
 
-    private class DiffCallback extends DiffUtil.Callback {
+    private static class DiffCallback extends DiffUtil.Callback {
         private List<TupleKeyword> prev = new ArrayList<>();
         private List<TupleKeyword> next = new ArrayList<>();
 

app/src/main/java/eu/faircode/email/AdapterLog.java

@@ -117,7 +117,7 @@ public class AdapterLog extends RecyclerView.Adapter<AdapterLog.ViewHolder> {
         diff.dispatchUpdatesTo(this);
     }
 
-    private class DiffCallback extends DiffUtil.Callback {
+    private static class DiffCallback extends DiffUtil.Callback {
         private List<EntityLog> prev = new ArrayList<>();
         private List<EntityLog> next = new ArrayList<>();
 

app/src/main/java/eu/faircode/email/AdapterNavAccount.java

@@ -229,7 +229,7 @@ public class AdapterNavAccount extends RecyclerView.Adapter<AdapterNavAccount.Vi
         diff.dispatchUpdatesTo(this);
     }
 
-    private class DiffCallback extends DiffUtil.Callback {
+    private static class DiffCallback extends DiffUtil.Callback {
         private List<TupleAccountEx> prev = new ArrayList<>();
         private List<TupleAccountEx> next = new ArrayList<>();
 

app/src/main/java/eu/faircode/email/AdapterNavFolder.java

@@ -204,7 +204,7 @@ public class AdapterNavFolder extends RecyclerView.Adapter<AdapterNavFolder.View
         diff.dispatchUpdatesTo(this);
     }
 
-    private class DiffCallback extends DiffUtil.Callback {
+    private static class DiffCallback extends DiffUtil.Callback {
         private List<TupleFolderNav> prev = new ArrayList<>();
         private List<TupleFolderNav> next = new ArrayList<>();
 

app/src/main/java/eu/faircode/email/AdapterNavMenu.java

@@ -169,7 +169,7 @@ public class AdapterNavMenu extends RecyclerView.Adapter<AdapterNavMenu.ViewHold
         return items.get(pos);
     }
 
-    private class DiffCallback extends DiffUtil.Callback {
+    private static class DiffCallback extends DiffUtil.Callback {
         private List<NavMenuItem> prev = new ArrayList<>();
         private List<NavMenuItem> next = new ArrayList<>();
 

app/src/main/java/eu/faircode/email/AdapterNavUnified.java

@@ -177,7 +177,7 @@ public class AdapterNavUnified extends RecyclerView.Adapter<AdapterNavUnified.Vi
         diff.dispatchUpdatesTo(this);
     }
 
-    private class DiffCallback extends DiffUtil.Callback {
+    private static class DiffCallback extends DiffUtil.Callback {
         private List<TupleFolderUnified> prev = new ArrayList<>();
         private List<TupleFolderUnified> next = new ArrayList<>();
 

app/src/main/java/eu/faircode/email/AdapterOperation.java

@@ -297,7 +297,7 @@ public class AdapterOperation extends RecyclerView.Adapter<AdapterOperation.View
         diff.dispatchUpdatesTo(this);
     }
 
-    private class DiffCallback extends DiffUtil.Callback {
+    private static class DiffCallback extends DiffUtil.Callback {
         private List<TupleOperationEx> prev = new ArrayList<>();
         private List<TupleOperationEx> next = new ArrayList<>();
 

app/src/main/java/eu/faircode/email/AdapterOrder.java

@@ -119,7 +119,7 @@ public class AdapterOrder extends RecyclerView.Adapter<AdapterOrder.ViewHolder>
         diff.dispatchUpdatesTo(this);
     }
 
-    private class DiffCallback extends DiffUtil.Callback {
+    private static class DiffCallback extends DiffUtil.Callback {
         private List<EntityOrder> prev = new ArrayList<>();
         private List<EntityOrder> next = new ArrayList<>();
 

app/src/main/java/eu/faircode/email/AdapterRule.java

@@ -489,7 +489,7 @@ public class AdapterRule extends RecyclerView.Adapter<AdapterRule.ViewHolder> {
         set(protocol, all);
     }
 
-    private class DiffCallback extends DiffUtil.Callback {
+    private static class DiffCallback extends DiffUtil.Callback {
         private List<TupleRuleEx> prev = new ArrayList<>();
         private List<TupleRuleEx> next = new ArrayList<>();
 

app/src/main/java/eu/faircode/email/AdapterRuleMatch.java

@@ -123,7 +123,7 @@ public class AdapterRuleMatch extends RecyclerView.Adapter<AdapterRuleMatch.View
         diff.dispatchUpdatesTo(this);
     }
 
-    private class DiffCallback extends DiffUtil.Callback {
+    private static class DiffCallback extends DiffUtil.Callback {
         private List<EntityMessage> prev = new ArrayList<>();
         private List<EntityMessage> next = new ArrayList<>();