code: def stop_app(self, callback_function_param=False):
          
          self.logger.info("Receiver:Stopping current app '%s'", self.app_id)
          return self.send_message(
              {MESSAGE_TYPE: 'STOP'},
              inc_session_id=True, callback_function=callback_function_param)
docs: Stops the current running app on the Chromecast.