Fix cancel notification race condition bug.

Cancel should always remove notifications, regardless of whether
they've been posted internally.  Primarily accomplished by ensuring
all cancels happen on the same handler, and that they check the
enqueued list as well the main notifications list.

Also fixes snoozing for enqueued notifications, since it is
implemented via cancelling.

Test: runtest systemui-notification
Change-Id: If8295a38102f245013d29831a060fabd80716e4b