MessageQueue.isBlockedOnSyncBarrier() fixes

 - add some missing checks for null references
 - we need to flush our trieber stack before checking
   for deliverable messages
 - mirror the changes into ConcurrentMessageQueue
   - also add some compile fixes to ConcurrentMessageQueue

Test: atest TestLooperManagerTest
Bug: 379472827
Flag: android.os.message_queue_testability
Change-Id: I1072117781ffe7765fb9c8f51b39f09829b8435f