Fix a T -> T restore issue

For pkgs that aren't preinstalled, we were erroneously
migrating notif settings to the permission system, which
was bad because we didn't have notif data in the backup to
migrate.

Test: PreferencesHelperTest
Test: manual - restore a T backup onto T, check B&R package notif settings
(not overwritten) & monitor logs - no processes killed because of denying
the permission
Bug: 218990774

Change-Id: I9fc0e8abeb6b1fbb54dfd4a5d45e6b3dabce04e9