vp9-svc: Fix key frame update refresh simulcast flexible svc
vp9-svc: Fix key frame update refresh simulcast flexible svc

For flexible svc in simulcast mode: don't allow refresh
of all reference slots on key frame. Which slots to update
should be based on the user flags.

Change-Id: I3597c61ebcdfed2055bbdffec7ce701fad892744

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