code: public void sendInBackground(final SendCallback callback) {
      sendInBackground().subscribe(new Observer<JSONObject>() {
        @Override
        public void onSubscribe(Disposable disposable) {
  
        }
  
        @Override
        public void onNext(JSONObject jsonObject) {
          notification = new AVObject("_Notification");
          notification.resetServerData(jsonObject.getInnerMap());
          if (null != callback) {
            callback.internalDone(null);
          }
        }
  
        @Override
        public void onError(Throwable throwable) {
          if (null != callback) {
            callback.internalDone(new AVException(throwable));
          }
        }
  
        @Override
        public void onComplete() {
  
        }
      });
    }
docs: Sends this push notification in a background thread. This is preferable to using send(), unless
  your code is already running from a background thread.
  
  @param callback callback.done(e) is called when the send completes.