BroadcastQueue: store "runnable" as linked list.

Our initial strategy was focused on implementation correctness, with
the known overhead of a sorted ArrayList.  This CL is a refinement
of that design that stores the "runnable" queues as a sorted linked
list, along with tests to confirm correctness.

Bug: 245771249
Test: atest FrameworksMockingServicesTests:BroadcastQueueTest
Test: atest FrameworksMockingServicesTests:BroadcastQueueModernImplTest
Change-Id: Ia431ce2e0505a26a72a82fc2b2bf50c45e41fbef