Add a BufferQueue CPU consumer.

Aimed for use cases where gralloc buffers need to be consumed by CPU
users, such as camera image data streams.

The CpuConsumer is a synchronous queue, which exposes raw pointers to
the underlying graphics buffers to applications. Multiple buffers may
be acquired at once, up to the limit set at time of construction.

Change-Id: If1d99f12471438e95a69696e40685948778055fd