DeliQueue: store sync barrier state in mWaitState

nextMessage() stores whether we are waiting on a sync barrier.
If enqueueMessageUnchecked() sees this bit as set it will skip
the wakeup call to the looper thread.

Test: atest MessageQueueTest
Bug: 421623328
Flag: EXEMPT new data structure isn't used yet; usages will be flagged.
Change-Id: I0e413e44a5d8dcfee6385af93becde4689f1e9d4