Revert "Updated paging library"

This reverts commit eae176136f.

app/build.gradle

@@ -242,7 +242,7 @@ dependencies {
     def lifecycle_version = "2.2.0" // 2.3.0-alpha05
     def sqlite_version = "2.1.0"
     def room_version = "2.2.5" // 2.3.0-alpha01
-    def paging_version = "3.0.0-alpha02"
+    def paging_version = "2.1.2" // 3.0.0-alpha02
     def preference_version = "1.1.1"
     def work_version = "2.4.0-rc01"
     def exif_version = "1.3.0-alpha01"

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

@@ -5447,7 +5447,7 @@ public class AdapterMessage extends RecyclerView.Adapter<AdapterMessage.ViewHold
         try {
             // https://issuetracker.google.com/issues/135628748
             Handler handler = new Handler(Looper.getMainLooper());
-            Field mMainThreadExecutor = this.differ.getClass().getDeclaredField("mainThreadExecutor");
+            Field mMainThreadExecutor = this.differ.getClass().getDeclaredField("mMainThreadExecutor");
             mMainThreadExecutor.setAccessible(true);
             mMainThreadExecutor.set(this.differ, new Executor() {
                 @Override