BroadcastQueue: offer to wait for a barrier.

The current "wait-for-idle" mechanism can get quite bogged down when
the system is under heavy load, leading to test timeouts.  Instead,
some clients are purely interested in knowing that all currently
outstanding broadcasts have been dispatched, so this change offers
a new "wait-for-barrier" mechanism.

Bug: 243656033
Test: atest CtsContentTestCases:BroadcastReceiverTest
Test: atest FrameworksMockingServicesTests:BroadcastQueueTest
Change-Id: Ie58a4111921b5b6f256904e002fe9fea12fc905a