Let Settings measure transient free space.

Transient volumes like USB drives are only mounted at /mnt/media_rw,
but they still appear in Settings > Storage.  To show stats like
free/used space, give Settings the permissions it needs to access
devices mounted there.

Bug: 22545248
Change-Id: I50be38f02f7d2bfe51d1ac0f76a04c340a21f936