Stops the current running app on the Chromecast.

def stop_app(self, callback_function_param=False):
        """ Stops the current running app on the Chromecast. """
        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)