Add auto-white balance locking to the Camera API.

Adds a new camera parameter for locking auto-white balance to its
current value. Also adds a function for checking if auto-white balance
lock is supported by the current platform. Lock semantics match that
of the auto-exposure lock.

Hidden for now.

Change-Id: Id59339a4be84d55c1b0b8473d765b6aa765999b2