vp8: Clean up skin detection.
vp8: Clean up skin detection.

Use only the average of center 2x2 pixels in vp8.

Change-Id: I2b23ff19a90827226273e0fca49e90c734eda59b

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