Use SATD based mode decision for block sizes below 16x16

Use SATD based mode decision for block sizes below 16x16

This commit makes the encoder to select between SATD/variance as
metric for mode decision. It also allows to account chroma
component costs for mode decision as well. The overall encoding
time increase as compared to variance based mode selection is about
15% for speed -6. The compression performance is on average 2.2%
better than variance based approach, with about 5% compression
performance gains for hard clips (e.g., jimredvga, nikas720p, and
mmmoving) at lower bit-rate range.

Change-Id: I4d04a31d36f4fcb3f5f491dacd6e7fe44cb9d815