Remove ExternalStorageProvider special case for having media_rw

ExternalStorageProvider needs to access unreliable USB OTG volumes.
It will have the MANAGE_EXTERNAL_STORAGE permission which will give it
the external_storage gid which will in turn give it access to
/mnt/media_rw/<volume> paths where 'volume' is a volume mount that is
not visible to other apps, so does not have a FUSE volume stacked
ontop.

Test: ExternalStorageProvider has the external_storage gid
Bug: 144914977
Change-Id: Ie68d3a5afcb137b034779137cdfbf4d8b6d60c47