Don't prevent notification updates in DOS protection.

The NotificationManagerService limits the number of
notifications per package+user to 50. Once that limit is
reached, it will refuse new notifications. However, it was
also erroneously refusing updates to existing notifications.

Bug: 5821364

(cherry picked from commit d9f4c95b)

Change-Id: I9abc455ef3863730f8e42c8eecff4892cdbc7c1c