For key frame at speed 6: enable the non-rd mode selection in speed setting
and use the (non-rd) variance_based partition.

Adjust some logic/thresholds in variance partition selection for key frame only (no change to delta frames),
mainly to bias to selecting smaller prediction blocks, and also set max tx size of 16x16.

Loss in key frame quality (~0.6-0.7dB) compared to rd coding,
but speeds up key frame encoding by at least 6x.
Average PNSR/SSIM metrics over RTC clips go down by ~1-2% for speed 6.

Change-Id: Ie4845e0127e876337b9c105aa37e93b286193405