Change gid bit of /mnt/media_rw to external_storage

To allow apps with MANAGE_EXTERNAL_STORAGE permission and therefore
external_storage gid to access unreliable volumes directly on
/mnt/media_rw/<volume>, they need access to the /mnt/media_rw path.

This change doesn't break the FUSE daemon, the only process that should
have media_rw gid in R because the FUSE daemon accesses the lower
filesystem from the pass_through bind mounts of the public volume mount
itself so it doesn't need to walk the /mnt/media_rw path itself

Test: With FUSE enabled, a reliably mounted public volume is accessible
on /storage
Bug: 144914977

Change-Id: Ia3fc9e7483894402c14fb520024e2acca821a24d