When applying a RankingMap that lacks an entry, remove that entry from the NotifCollection.

This fixes a bug after notification removed messages were dropped during moments of mass removal.  The NotifCollection was detecting this edge case but ignoring it.  Now we'll actually remove the notification and require an add event to get it back.

Note: this feature is still flagged off by default.

Bug: 216384850
Test: atest NotifCollectionTest
Change-Id: Id897c8761e40fd1adedd7c9e5a7bd2f9467d13ed