BroadcastQueue: implement ordered broadcasts.

This change implements ordered broadcasts in the modern queue in as
graceful a way as possible.  When the "head" of a process queue is
an ordered broadcast, and it's not the next-in-line, it demotes
itself to be not runnable.  Later, as each ordered receiver is
finished, we reevaluate the process queue of the next ordered
receiver to see if they're now runnable.

We could try some fancy footwork when a broadcast is aborted to
immediately fast-forward to delivering resultTo, but it's safest if
we let the remainder of the ordered broadcast proceed and handle
each receiver as "skipped".  (If we tried fast-forwarding, we'd
have a tangled mess of things to update like mCountOrdered.)

Also add more docs.

Bug: 245771249
Test: atest FrameworksMockingServicesTests:BroadcastQueueTest
Change-Id: Icc63905175b15707e9e1414c3b3e994b4382304b