vp9-rtc: Adjust thresh for 4x4 tx selection
vp9-rtc: Adjust thresh for 4x4 tx selection

For screen content nonrd_pickmode: reduce
threshold to select 4x4 tx_size, under certain
conditions.

Change-Id: If68c30172272868033f0e3011e53c76b4e7c48b6

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