Add external_storage gid

This gid allows processes full access to public areas of external
storage. This includes the following:
1. EmulatedVolumes: All files and directories excluding the app
specific directories under Android/
2. PublicVolumes: Including 'unreliable' volumes (USB OTG) that are
not typically accesible to ordinary apps

Apps with the MANAGE_EXTERNAL_STORAGE permission will automatically
have this gid

Test: m
Bug: 144914977
Change-Id: I17da0b2367e356edc031d063e214574463afc985