vp9-svc: Adjust search step param for spatial layers
vp9-svc: Adjust search step param for spatial layers

For non-base spatial layer in screen-content mode:
use nstep but with larger step_param value than sl0,
to avoid increase in encode_time.
Some improvement on scrolling slides content.

Change-Id: Ica918ac01664431d1fabb3c674d857cf6ad87414

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