Fix tx type as the default type and search for the best prediction mode;
then do a final search for the best tx type.

When it's enabled at speed 0, encoding speed increases to 3x with 2%
compression loss on keyframes. It's currently turned on for speed
2 and above.

Change-Id: Ic9ca1479da10d467a88a4d692d9c75da40d2798d