Replace copy_partitioning use case with choose_partitioning

This commit replaces the use of copy_partitioning with
choose_partitioning based on the sse of subsamped pixels, which
provides significantly better coding performance and runs at
similar speed, as compared to copy_partitioning. It improves rtc
speed 5 coding performance by 3%.

Change-Id: I52d3682a12dce0147f5e52383a594fc242ca3228
Merge "Replace copy_partitioning use case with choose_partitioning"