[8/?] Reduce BroadcastQueue interface complexity.

Remove visibility of "pending" and "active" broadcasts, since these
are details of the current implementation.  A future implementation
will likely pivot to having multiple broadcasts in flight at the same
time, so we replace them with getPreferredSchedulingGroupLocked(),
which is the real question that the OS is asking.

Bug: 243656033
Test: atest CtsContentTestCases:BroadcastReceiverTest
Test: atest FrameworksMockingServicesTests:BroadcastQueueTest
Change-Id: I1b567b124d332803cc79f2fe77ecd89d410ff393