BroadcastQueue: better state transition logging.

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

Register Loopers that are well-known to be used for broadcast
dispatch so that we fully drain them before "wait-for-broadcast".

Remove outdated TODOs that are either finished or tracked elsewhere,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: I7fe33cd173919f98c062c488598f81ac732c52a3