Add log to NoMan for canceled cancels

In the case where a notification_cancel is skipped when finding a more
recently enqueued notification for the same key, now throw a log so we
know that we're dropping a message from the system.

Bug: 162652224
Test: manual
Change-Id: I313076f8b15b00286f532ace1bbe7da2f7db4afe
1 file changed