+ * @remark You must not free returned widget instance id
+ * @remark The returned widget instance id is volatile. If the device reboots or the widget's process restarts, it will be changed.\n
+ * So, you should not assume this value is a persistent one.
+ * @remark widget_service_trigger_update(), widget_service_change_period(), widget_service_get_content_of_widget_instance()\n
+ * can be called with returned instance id.