vp9-rtc: Increase thresh for scene detection
vp9-rtc: Increase thresh for scene detection

For CBR screen content mode. Makes it more
robust to false detections.

Change-Id: Icad89adb6f79b530b589bba2c71ba88ee5088d37

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