Camera3: add dynamic buffer count water mark support

The dynamic buffer count water mark starts from zero, and grows when the max of
hand-out buffer counts grow, until reach to the max allowed total buffer count.
For the case where the clients almost always uses less number of buffers than
the max allowed total buffer count, the dynamic buffer count water mark will
remain low, and the memory footprint will be smaller.

Change-Id: I305f40232f4740d3e9bedf14ee9b76a81e29e244