Run all AsyncTasks on thread pool executor

Should Fix #7002

src/org/thoughtcrime/securesms/util/IdentityUtil.java

@@ -62,7 +62,7 @@ public class IdentityUtil {
       protected void onPostExecute(Optional<IdentityRecord> result) {
         future.set(result);
       }
-    }.execute(recipient);
+    }.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, recipient);
 
     return future;
   }

src/org/thoughtcrime/securesms/util/Trimmer.java

@@ -12,7 +12,7 @@ import org.thoughtcrime.securesms.database.ThreadDatabase;
 public class Trimmer {
 
   public static void trimAllThreads(Context context, int threadLengthLimit) {
-    new TrimmingProgressTask(context).execute(threadLengthLimit);
+    new TrimmingProgressTask(context).executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, threadLengthLimit);
   }
 
   private static class TrimmingProgressTask extends AsyncTask<Integer, Integer, Void> implements ThreadDatabase.ProgressListener {

src/org/thoughtcrime/securesms/util/task/SnackbarAsyncTask.java

@@ -85,7 +85,7 @@ public abstract class SnackbarAsyncTask<Params>
           progressDialog = null;
         }
       }
-    }.execute();
+    }.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
   }
 
   protected abstract void executeAction(@Nullable Params parameter);