Adjustment to scene detection and key frame.

Adjustment to scene detection and key frame.

For 1 pass vbr: use higher threshold on avg_sad
and force key frame under scene cut detection if
above the threshold. Allow it for speed >= 6 for now,
since it does not use the full nonrd_pickmode partition
(as in speed 5).

Improves quality somewhat on scene cut frames.
Neutral on overall metrics and fps for speed 6 on
ytlive set.

Change-Id: I12626f7627419ca14f9d0d249df86c7104438162