Correct the setting of Q basis of GF/ARF in 1 pass vbr.

Existing logic would switch to using avg_QP of key frame if
avg_QP of inter is less than active worst (even if key frame is
not last frame).

Instead fix the logic (as per the comment) to use the lower of
active_worst_quality and avg_Q for inter as basis for GF/ARF
active_best_quality (unless last frame was key frame).

Increase in metrics: AvgPSNR/SSIM up by ~0.7/0.3 on ytlive set.

Change-Id: I9a628378ec6684bfda9457ebfc2384ef6d8579f7