BroadcastQueue: DISPATCH_SENDING_BROADCAST_EVENT.

Consistently send this event from both legacy and modern stack, and
refactor to hide the DISPATCH_SENDING_BROADCAST_EVENT message as an
implementation detail inside AMS.

Tests to verify that we always deliver expected "finished" events.

Bug: 257038590
Test: atest FrameworksMockingServicesTests:BroadcastRecordTest
Test: atest FrameworksMockingServicesTests:BroadcastQueueTest
Test: atest FrameworksMockingServicesTests:BroadcastQueueModernImplTest
Change-Id: Ib5c0b78ede454a11cac75cec06deebc6adbb8a40