BufferQueueProducer: Throttle EGL frame production.

Throttling was previously controlled by a combination of the
driver and the number of buffers in the queue. This patch makes
a more consistent trade-off, which allows two GPU frames pending
but not three. More buffering could improve throughput in the
case of varying frame times, but this also increases latency.

Bug: 17502897
Change-Id: I4ee68019ca94c635294c5959931a555a6c4ef2df