Log assistant notification cancels accordingly.

This change adds the NOTIFICATION_CANCEL_ASSISTANT uievent and reports any cancellations due to assistants as REASON_ASSISTANT_CANCEL. Listeners below T (when this reason is added) are notified with the same reason that was previously used in all cases, REASON_LISTENER_CANCEL.

Bug: 201417230
Test: NotificationManagerServiceTest, manually by statsd_testdrive

Change-Id: Iae50053d325b62b479b3c31aea97d35d8847f381