Use TestLooperManager to improve test robustness.

Many tests have already paused broadcast stack internals to get a
consistent view, so let's expand that to pause-by-default, and only
unpause when a test is interested in observing side-effects via
calls like waitForIdle().

TestLooperManager is the official public API mechanism to support
this, instead of abusing sync barriers.

Bug: 277947400
Test: atest FrameworksMockingServicesTests:BroadcastQueueTest
Test: atest FrameworksMockingServicesTests:BroadcastQueueModernImplTest
Change-Id: I19ea29e037aeebbdad4683c28ed137204a60efa6