BroadcastQueue: better state transition logging.

To aid future debugging of the modern stack, include a "reason"
behind each state transition.

Remove outdated TODOs that are either finished or tracked elsewhere,
add misc annotations.

Bug: 254521331
Test: atest FrameworksMockingServicesTests:BroadcastRecordTest
Test: atest FrameworksMockingServicesTests:BroadcastQueueTest
Test: atest FrameworksMockingServicesTests:BroadcastQueueModernImplTest
Change-Id: I307dc86b1935d5ca88a8ca20869cb9120e05bf12