Permission to view shared storage for all users.

Typical apps are restricted so they can only view shared storage
belonging to the user they're running as.  However, a handful of
system components need access to shared storage across all users,
such as DefaultContainerService and SystemUI.

Since WRITE_MEDIA_STORAGE already offers this functionality by
bypassing any FUSE emulation, reuse it to grant the "sdcard_rw" GID
which is no longer handed out to third-party apps.  Then we change
the FUSE daemon to allow the "sdcard_rw" GID to see shared storage
of all users.

Bug: 19995822
Change-Id: I504c2a179ba74f142ed0d32da5baa69f4212cd82