Enable "modern" BroadcastQueue by default; take three.

We've completed several weeks of teamfooding which has helped us
gain confidence in the "modern" queue, so let's pivot it enabling it
by default.

Bug: 253906105
Test: atest FrameworksMockingServicesTests:BroadcastRecordTest
Test: atest FrameworksMockingServicesTests:BroadcastQueueTest
Test: atest FrameworksMockingServicesTests:BroadcastQueueModernImplTest
Change-Id: Ic51abda906a37c0fa19729e45ec1d8e35fcc95b6