BroadcastQueue: add support for wait-for-barrier.

We recently added this feature to help tests finish faster without
needing to wait for complete idle, and this change expands support
to the "modern" queue, along with tests.

Fix bugs in "default" implementation where we need to consult all
pending broadcasts in the dispatcher.

Bug: 245771249
Test: atest FrameworksMockingServicesTests:BroadcastQueueTest
Test: atest FrameworksMockingServicesTests:BroadcastQueueModernImplTest
Change-Id: I9be5412b1d6020212ceae674d9f3685c3bdec3c7