Android系统应用不支持读写存储

Android 加了android.uid.system后，读写Storage会报错：

EACCES (Permission denied)

后来查到有说：

The system user can not access the SD card, because if the SD card gets unmounted it may need to kill any processes that have files open on it and we don’t want system processes being killed like that. If you want to access the SD card, you need to not use the system shared user ID.