Run async differ on background thread

You keep telling developers to not run things in foreground threads,
so why-o-why Google isn't this the default?

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

@@ -145,6 +145,8 @@ import java.util.Locale;
 import java.util.Map;
 import java.util.Objects;
 import java.util.concurrent.Executor;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
 
 import javax.mail.Address;
 import javax.mail.internet.InternetAddress;
@@ -224,6 +226,9 @@ public class AdapterMessage extends RecyclerView.Adapter<AdapterMessage.ViewHold
     private DateFormat TF;
     private DateFormat DTF;
 
+    private static final ExecutorService executor =
+            Executors.newFixedThreadPool(2, Helper.backgroundThreadFactory);
+
     // https://github.com/newhouse/url-tracking-stripper
     private static final List<String> PARANOID_QUERY = Collections.unmodifiableList(Arrays.asList(
             "utm_source",
@@ -3586,6 +3591,7 @@ public class AdapterMessage extends RecyclerView.Adapter<AdapterMessage.ViewHold
         debug = prefs.getBoolean("debug", false);
 
         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>() {