Keep read/write external storage as built-in permissions

These are permissions that were mapped to gids but we need
to keep them listed event though they are no longer mapped
to gis until an upgrade from L to the current version is to
be supported. These permissions are built-in and in L were
not stored in packages.xml as a result if they are not defined
in the platform.xml while parsing packages.xml we would
ignore these permissions being granted to apps and not
propagate the granted state. From N we are storing the built-in
permissions in packages.xml as the saved storage is negligible
(one tag with the permission) compared to the fragility as one
can remove a built-in permission which no longer needs to be
mapped to gids and break grant propagation.

bug:27185272

Change-Id: I440f6ceb7bc4710dece1a2fadabc995b18fc2a83