Defer FGS notification display for a few seconds

When a service is transitioned to the foreground state, we defer display
of its associated Notification for a short time, to reduce user
disturbance in situations where the FGS is short-lived.  Apps can force
immediate display when they know it's relevant, and Notifications known
to correspond to contexts in which immediate display is appropriate -
such as media playback - are not deferred.

The behavior can be disabled or the deferral interval adjusted via
DeviceConfig.

Bug: 171499612
Test: ApiDemos
Test: atest CtsAppTestCases:ServiceTest
Test: atest CtsAppTestCases:NotificationManagerTest
Change-Id: I0cae3dc6f943e99873ed8c1687914ad08ec29b57