More tests for BroadcastRecord.

This change moves the existing BroadcastRecordTest into a consistent
suite location with the other broadcast-related tests, and then adds
tests for recently added logic.

Clearly annotate nullness of fields.

Bug: 250666387
Test: atest FrameworksMockingServicesTests:BroadcastRecordTest
Test: atest FrameworksMockingServicesTests:BroadcastQueueTest
Test: atest FrameworksMockingServicesTests:BroadcastQueueModernImplTest
Change-Id: I18847c61ea7b74c4324abf51005b61728dd2acb7