libvpx

Modify active_worst_quality setting for one pass CBR.

Current setting had active_worst_quality set too high (close to worst_quality)
for first frame(s) following first key frame. This changes that to be somewhat
more aggressive in allowing active_worst_quality to be lower following key frame.

Also remove the 4/5 reduction in active_worst for key frame as
this should be set by the user qp_max setting.

Change-Id: I0530b3ddcc85c00e3eb7568de1b14a31206c4a4c