Force FGS notifications to show for a minimum time

It's possible for a service to do a start/stop foreground and cause a
couple of things to happen:

NotificationManagerService will enqueue a EnqueueNotificationRunnable,
post a PostNotificationRunnable (for the startForeground), and then also
enqueue a CancelNotificationRunnable. There is some racy behavior here
in that the cancel runnable can get triggered in between enqueue and
post runnables. If the cancel happens first, then
NotificationListenerServices will never get the message.

This behavior is technically allowed, however for foreground services we
want to ensure that there is a minmum amount of time that notification
listeners are aware of the foreground service so that (for instance) the
FGS notification can be shown.

This CL does two things to mitigate this problem:

1. Introduce checking in the CancelNotificationRunnable such that it
will not cancel until after PostNotificationRunnable has finished
executing.

2. Introduce a NotificationLifetimeExtender method that will allow a
lifetime extender to manage the lifetime of a notification that has been
enqueued but not inflated yet.

Bug: 119041698
Test: atest NotificationManagerServiceTest
Test: atest ForegroundServiceNotificationListenerTest
Change-Id: I0680034ed9315aa2c05282524d48faaed066ebd0