vp9-svc: Set avg_frame_low_motion for lower layers.
vp9-svc: Set avg_frame_low_motion for lower layers.

The avg_frame_low_motion metric is only computed on the
top spatial layer, and since its part of the layer context
struct, it needs to written to all lower spatial layers for
consistency.

Small/minor change in metrics.
Change-Id: I92a001c37aeb332e613212288b13a2ed9745af88

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