Run all AsyncTasks on thread pool executor

Should Fix #7002

src/org/thoughtcrime/securesms/scribbles/ScribbleActivity.java

@@ -165,7 +165,7 @@ public class ScribbleActivity extends PassphraseRequiredActionBarActivity implem
             protected void onPostExecute(@Nullable Bitmap bitmap) {
               addSticker(bitmap);
             }
-          }.execute();
+          }.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
         }
       }
     }

src/org/thoughtcrime/securesms/scribbles/widget/ScribbleView.java

@@ -135,7 +135,7 @@ public class ScribbleView extends FrameLayout {
         canvasView.render(canvas);
         future.set(bitmap);
       }
-    }.execute();
+    }.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
 
     return future;
   }