Grant dev tools system permission to trigger media-scan service

Google add the action ACTION_MEDIA_MOUNT into the list of
protected boradcast, so that ordinary apps cannot trigger
media scanning by directly sending the action out. The system
built-in dev tool is granted system permission so that it
will not crash when executing media scaning tools.

Change-Id: I19797e61ba829df39d2b66f209cf41bf6565170f