Grant ExternalStorageProvider media_rw

After deprecating the WRITE_MEDIA_STORAGE permission,
ExternalStorageProvider no longer has access to /mnt/media_rw. This
access is necessary to allow SAF access 'unstable' (USB OTG) volumes.

We manually grant the media_rw permission to ExternalStorageProvider

Test: cat /proc/<pid>/status shows media_rw
Bug: 144914977
Change-Id: I8d991fbeb1f144c3f45143e6cafce7bf140081fa