BufferQueue: Guard against unbounded queue growth

Adds logic to dequeueBuffer that blocks if there are currently too
many buffers in the queue. This prevents unbounded growth around
times where the slots are cleared but the queue is not (e.g.,
during rapid connect/disconnect or setBufferCount activity). This
replaces the fix from ag/377958 in a more general way.

Bug: 11293214
Change-Id: Ieb7adfcd076ff7ffe3d4d369397b2c29cf5099c3