Use background executor for paged message list

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

@@ -93,6 +93,8 @@ import androidx.paging.PagedList;
 import androidx.preference.PreferenceManager;
 import androidx.recyclerview.selection.ItemDetailsLookup;
 import androidx.recyclerview.selection.SelectionTracker;
+import androidx.recyclerview.widget.AdapterListUpdateCallback;
+import androidx.recyclerview.widget.AsyncDifferConfig;
 import androidx.recyclerview.widget.DiffUtil;
 import androidx.recyclerview.widget.LinearLayoutManager;
 import androidx.recyclerview.widget.RecyclerView;
@@ -118,6 +120,8 @@ import java.util.Arrays;
 import java.util.Collections;
 import java.util.Date;
 import java.util.List;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
 
 import javax.mail.Address;
 import javax.mail.internet.InternetAddress;
@@ -181,6 +185,8 @@ public class AdapterMessage extends RecyclerView.Adapter<AdapterMessage.ViewHold
     private DateFormat TF;
     private DateFormat DTF = SimpleDateFormat.getDateTimeInstance(SimpleDateFormat.LONG, SimpleDateFormat.LONG);
 
+    private static ExecutorService executor = Executors.newCachedThreadPool(Helper.backgroundThreadFactory);
+
     private static final List<String> PARANOID_QUERY = Collections.unmodifiableList(Arrays.asList(
             "utm_source",
             "utm_medium",
@@ -3447,7 +3453,10 @@ public class AdapterMessage extends RecyclerView.Adapter<AdapterMessage.ViewHold
         this.authentication = prefs.getBoolean("authentication", false);
         this.debug = prefs.getBoolean("debug", false);
 
-        this.differ = new AsyncPagedListDiffer<>(this, DIFF_CALLBACK);
+        AsyncDifferConfig<TupleMessageEx> config = new AsyncDifferConfig.Builder<>(DIFF_CALLBACK)
+                .setBackgroundThreadExecutor(executor)
+                .build();
+        this.differ = new AsyncPagedListDiffer<>(new AdapterListUpdateCallback(this), config);
         this.differ.addPagedListListener(new AsyncPagedListDiffer.PagedListListener<TupleMessageEx>() {
             @Override
             public void onCurrentListChanged(@Nullable PagedList<TupleMessageEx> previousList, @Nullable PagedList<TupleMessageEx> currentList) {