BroadcastQueue: add support for priority tranches.

Both manifest and registered receivers can specify a "priority" in
which they'd like to be delivered.  This can be used to ensure that
system components receieve a broadcast (and can update their internal
state) before third-party apps receive that broadcast.

The "default" stack handled this by dispatching all receivers in
the order defined by ActivityManagerService, but the "modern" stack
now needs to carefully only allow parallel dispatch within a specific
tranche of consistent priority.  (That is, we must finish dispatching
all receivers with priority "10" before moving on to priority "9".)

This change repurposes the logic we built for ordered broadcasts,
where a process queue is "blocked" while waiting on a certain number
of prior broadcasts to reach a terminal state.

To avoid priority inversions (where a lower priority receiver would be
blocked behind a higher priority receiver in a frozen state), we
currently promote all receivers waiting on a "prioritized" broadcast
to be immediately runnable.

Bug: 245771249
Test: atest FrameworksMockingServicesTests:BroadcastQueueTest
Test: atest FrameworksMockingServicesTests:BroadcastQueueModernImplTest
Change-Id: Ic6ff9a959a262bd73ecd3d7977f400d10b81535d