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.

avc: denied { search } for name="media_rw" dev="tmpfs" ino=8358 scontext=u:r:system_app:s0 tcontext=u:object_r:mnt_media_rw_file:s0 tclass=dir permissive=0

Bug: 22545248
Change-Id: I273a1729e417873184ad04ba9dd0fec95fd54f97