cameraservice: decrease zsl metadata queue size by 1

Zsl buffer needs to be longer than metadata queue to ensure that
oldest metadata can always find a match in buffer queue.
Since we don't want to add memory overhead, decrease metadata
queue size by one serves the same purpose.

bug 17264283

Change-Id: Ic53441cc29c98e57d3345f5845d92839d0ce6faf