BufferQueue: use max acquired buffer count

This change makes BufferQueue derive the min undequeued buffer count from a max
acquired buffer count that is set by the consumer.  This value may be set at
any time that a producer is not connected to the BufferQueue rather than at
BufferQueue construction time.

Change-Id: Icf9f1d91ec612a079968ba0a4621deffe48f4e22