DO NOT MERGE: Associate notif cancels with notif posts

CancelNotificationRunnables just spin on the work handler of
NotificationManagerService, hoping that they get executed at the correct
moment after a PostNotificationRunnable and before the next
EnqueueNotificationRunnable completes. Otherwise, you end up in a bad
state where the cancel either is canceling notifications before they get
a chance to post, or missing its only chance to cancel the notification
(for instance, ActivityManagerService is the only caller that can cancel
FGS notifications).

This change attempts to execute a CancelNotificationRunnable at the
moment its run() method is called, otherwise it associates the runnable
with the latest enqueued notificaiton record which has yet to post.

It then associates PostNotificationRunnable with the delayed cancel
list, executing any missed cancel operations immediately upon finishing
the PostNotificationRunnable.

Test: atest SystemUITests; manual
Bug: 162652224
Change-Id: I88d3c5f4fd910a83974c2f84ae3e8a9498d18133