[9/?] Reduce BroadcastQueue interface complexity.

Adds some additional methods to BroadcastRecord to aid in upcoming
per-process queue changes.

Add UidObserver with default no-op implementation to aid readability
of child classes.

Updates unit tests to acquire relevant locks.

Bug: 243656033
Test: atest CtsContentTestCases:BroadcastReceiverTest
Test: atest FrameworksMockingServicesTests:BroadcastQueueTest
Change-Id: I2ff1e868586861e4dcd6586ad22139ba84eaf39c