Make FGS notification deferral control tri-state

Apps can now request either immediate visibility or deferral explicitly,
versus the previous iteration's immediate-or-default only.

Bug: 179290175
Test: ApiDemos foreground service exercise
Test: atest CtsAppTestCases:ServiceTest
Test: atest CtsAppTestCases:NotificationManagerTest
Change-Id: I0b4d11a7483d2407758c810cf4a77a2e45bb737f