Improve MessageQueue sync barrier implementation.

Instead of acquiring and releasing a barrier using an up/down
counter, we post a message to the queue that represents the
barrier.  This is a more natural representation of the barrier
and better matches what we want to do with it: stall messages
behind the barrier in the queue while allowing messages earlier
in the queue to run as usual.

Refactored the MessageQueue a little bit to simplify the quit
logic and to better encapsulate the invariant that all
messages within the queue must have a valid target.  Messages
without targets are used to represent barriers.

Bug: 5721047
Change-Id: Id297d9995474b5e3f17d24e302c58168e0a00394