vp9; Reduce artifact in non-rd pickmode for lighting changes.

vp9; Reduce artifact in non-rd pickmode for lighting changes.

Add a low-variance high-sumdiff to the superblock content state
and use it to limit the mv and bias some decisions in non-rd pickmode.
Only affects speed >= 6.

Reduces artifact for lighting changes.
Small/no difference in metrics on RTC set.

Change-Id: Ic84b2379fe0ae3fa71ae826ee6bae3eaf551a25b