Add a barrier mechanism to the MessageQueue.

The synchronization barrier enables selectively blocking
execution of synchronous messages until the barrier is released.
Asynchronous messages may continue running in the meantime.

The barrier is intended to be used to implement more sophisticated
scheduling policies related to view hierarchy traversals.  While
traversals are pending, most messages posted to the message queue
must be held up.  This is to satisfy the invariant that traversals
will occur before subsequently posted messages are handled.

The exception to this rule are "asynchronous" messages that represent
external events or interrupts that come from other components such
as VSYNC pulses, input events or sensor events.  Because these messages
are typically delivered at arbitrary times, they are independent of
traversals or other typical synchronization boundaries.

Messages can now be flagged as asynchronous to indicate that they
are weakly ordered.

Bug: 5721047
Change-Id: I1446dcfbc896f33b48355adc28967ace8c8c9b9b