Strip POST_NOTIFICATIONS from requests for pre-T apps

If a pre-T app requests POST_NOTIFICATIONS, strip the permission from
the request, exluding it from the returned results

Test: atest NotificationPermissionTest#legacyAppCannotExplicitlyRequestNotifications
Fixes: 222370223
Change-Id: I101be29cd00b3b6cfc36731749a7728bb5a0c3f7