Optofidelity: More reliable screen calibration

The screen calibration code has been updated with new logic to pick
reference frame from the video of flashing frames. The new logic
picks frames at the time when the direction of color change is flipping,
which turns out to be much more reliable in varying lighting settings.
The Validate method has also been updated to check the white level of
the calibration to make sure the calibration is neither under nor
over exposed.

BUG=chromium:505519
TEST=None

Change-Id: If1c063cbe292b5166f0bff7c49690b6742abf9e1
Reviewed-on: https://chromium-review.googlesource.com/336463
Commit-Ready: Dennis Kempin <denniskempin@chromium.org>
Tested-by: Dennis Kempin <denniskempin@chromium.org>
Reviewed-by: Dennis Kempin <denniskempin@chromium.org>