Introduce preserveLegacyExternalStorage manifest attribute

This attribute will default to false and does nothing in the false
state.

In the true state, it will allow an updating app preserve any legacy
storage access from the existing installed version on device.

Test: atest RestrictedPermissionsTest
Test: atest RestrictedStoragePermissionSharedUidTest
Bug: 148944140
Change-Id: Id5b6d7aadd9d9844c8bd1164087a9e093481e234