vp9: Update to skin detection.
vp9: Update to skin detection.

For model=1, use higher threshold on luminance, conditioned
on the skin cluster distance.

Change-Id: I971bccdef7b92b2aeef60af90a8e48b98716a89b

GrokImageCompression/aom,webmproject/libvpx,ittiamvpx/libvpx-1,mwgoldsmith/vpx,mwgoldsmith/libvpx,mbebenita/aom,luctrudeau/aom,mwgoldsmith/libvpx,GrokImageCompression/aom,Topopiccione/libvpx,ittiamvpx/libvpx-1,Topopiccione/libvpx,smarter/aom,webmproject/libvpx,mbebenita/aom,mwgoldsmith/vpx,mbebenita/aom,mbebenita/aom,mbebenita/aom,webmproject/libvpx,Topopiccione/libvpx,luctrudeau/aom,GrokImageCompression/aom,mbebenita/aom,mbebenita/aom,luctrudeau/aom,mbebenita/aom,ShiftMediaProject/libvpx,GrokImageCompression/aom,Topopiccione/libvpx,mwgoldsmith/libvpx,smarter/aom,luctrudeau/aom,mwgoldsmith/vpx,webmproject/libvpx,ShiftMediaProject/libvpx,GrokImageCompression/aom,Topopiccione/libvpx,GrokImageCompression/aom,ittiamvpx/libvpx-1,ShiftMediaProject/libvpx,smarter/aom,webmproject/libvpx,smarter/aom,ittiamvpx/libvpx-1,luctrudeau/aom,ittiamvpx/libvpx-1,Topopiccione/libvpx,mwgoldsmith/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/vpx,mwgoldsmith/libvpx,mbebenita/aom,smarter/aom,ittiamvpx/libvpx-1,ShiftMediaProject/libvpx,smarter/aom,mwgoldsmith/libvpx,mwgoldsmith/vpx,luctrudeau/aom,mwgoldsmith/vpx,webmproject/libvpx