Add improved_mv_pred flag in real-time mode

As mentioned in check-in "Improve motion search in real-time mode",
MV prediction calculation causes speed loss for speed 7 and above.
This change added a flag to turn off this calculation for speed>6
in real-time mode.

Change-Id: I9f4ae5a8bf449222d1784b54e7d315fc8347b2d1