Throttle camera preview frames to the app. Bug 2180302.

With higher frame rates and larger preview frames, we can easily
flood the application with too much data. This patch fakes the
old camera preview mode by doing continuous one-shot frames.
After the previous frame is handled, if the application hasn't
cleared the preview callback, we start another one-shot preview
frame. With this change, the application should never have more
than one preview frame unless it is explicitly saving references
to them.

	modified:   core/java/android/hardware/Camera.java