vp9: Adjust minmax threshold for variance partitioning.
vp9: Adjust minmax threshold for variance partitioning.

Only affects speed 7. Improvement on high motion clips.

Change-Id: Ibddb68fed9c63207df29ffd790f9205b1cecf687

webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx