Ignore nonsensical clears from NoMan

It appears that NoMan sometimes fails to send onNotificationPosted
events for some notifications (possibly due to the notification limit?).
It's possible to still receive onNotificationRemoved events for those
notifications, so we ignore those events for now (long-term we should
figure out if this is indeed a problem in NoMan).

Bug: 159652654
Test: atest
Change-Id: Ib15320f537e31a675d2766ba458b7bd019764c73