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: I65e05c4f7edd9a934888b4d0974100aa4e9a9453