Fix background bypass via notifications

Apps were able to bypass BAL and BG-FGS restrictions by retrieving their
own notifications and firing their PI since those were allowlisted for
those operations.

Now we strip the token that granted them that ability
from notifications returned via NM.getActiveNotifications(), which
returns the notifications of the caller.

Notifications returned via notification listener APIs still contain such
token, as they should.

Bug: 185388103
Bug: 169821287
Test: atest -d android.app.cts.NotificationManagerTest#testActivityStartFromRetrievedNotification_isBlocked
Change-Id: I2ede0d639a560f6acacec3864a0a7d23af152ba5