Upgrade NoMan's cancelation reposting logic

1. Technically there can be multiple notifications enqueued which
haven't posted yet. Check the entire set of them instead of just the
first.
2. If there is a posted notification that showed up _after_ the request
to cancel, then cancel the cancel (don't cancel is what I'm saying)

Bug: 147420515
Test: atest NotificationManagerServiceTest
Change-Id: I0f401eb3cc6b8c15fb34593f7afc3293daed0726