BroadcastQueue: tests for dispatch ordering.

Confirm that we always prefer running pending items in "urgent", then
"normal", then "offload" ordering, dispatching by the relative
ordering within each of those clustering groups.

Bug: 253906105
Test: atest FrameworksMockingServicesTests:BroadcastRecordTest
Test: atest FrameworksMockingServicesTests:BroadcastQueueTest
Test: atest FrameworksMockingServicesTests:BroadcastQueueModernImplTest
Change-Id: Iabeefa5d4449a581e4380209370ab2df81c8df5f