Fix MANAGE_EXTERNAL_STORAGE permission gid mapping

In Android R, we introduced a platform.xml based
mapping. Unfortunately, it only worked for signature|preinstalled
apps.

To support apps granted the appop (via special app access
permissions), we now check the permission and appop grant state
explicitly and grant the app the external_storage gid appropriately

Test: Manual
Bug: 165515144

Change-Id: Ib91e1b3a7e54ac2c83fb1d94446bed06fd44bcf6