The source sad could be used to copy the partition without going into
choose_partitioning function to speed up vp9 encoding. Computing source
sad takes little time. Speed test on Android and Linux shows little
encoding time gain (less than 1.4%).

Turned off for now since partition copy is turned off.

Change-Id: I61c9d5b8f22329760cb29a4ee30a7f9c232ce8d3