Statsd logging of notification cancels.

Logs notification cancel only if the notification was in fact posted.

Uses the UiEventReported framework.  Tweaked that framework to handle
null InstanceIds better.

Bug: 146488473
Test: atest NotificationManagerServiceTest BuzzBeepBlinkTest RoleObserverTest
Test: statsd_testdrive 90 244
Change-Id: Ifd6946ce64c3a09f8cac436bb7087ec56521486c