Fix issue when QUEUE_DESTROY arrive at the same time.

When multiple QUEUE_DESTROY messages arrive in a short timespan, some
messages were not flushed. This is because every QUEUE_DESTROY message
resets mFlushAll just before it starts playing itself. The solution
is to keep a counter of the amount of flushes that are still behind it on
the queue and keep flushing until the last one is reached.

Bug: 27555444
Change-Id: Ic24aa2250bf684e74b541907e4c57e0d5f0069ca