Avoid races between nextMessage() and stackHasMessages()

Always check if the stack is draining before we exit stackHasMessages().
 - This will ensure that next has put any pending messages into the
   priority queues before we search them.

Bug: 396569494
Flag: EXEMPT - bugfix
Test: atest MessageQueueTest
Change-Id: If3259e13060baae4125a4977519a7f08390f79cb