Testing strategy for generic BroadcastQueue.

Now that we have a reasonably clean BroadcastQueue interface, let's
start wiring up some basic local unit tests.  These tests will help
us rapidly verify that a suite of BroadcastQueue implementations all
behave consistently, without relying on heavy integration tests.

Bug: 243656033
Test: atest CtsContentTestCases:BroadcastReceiverTest
Test: atest FrameworksMockingServicesTests:BroadcastQueueTest
Change-Id: I58808888b87323be6e12d1fc92180f70b13d9a78