vp9 svc frame drop: enable adaptive rd for row mt.

vp9 svc frame drop: enable adaptive rd for row mt.

adaptive_rd_threshold_mt is set to 1 when speed >= 7 for SVC.
QVGA in SVC uses speed 5 which set adaptive_rd_threshold_mt to 0.
If VGA or HD is dropped for the last super frame, the flag is still 0
when the encoder is destroyed. Thus memory won't be released.

Change the bitrate threshold in datarate test.

Change-Id: I55352cc0b030568d38eb735d99c2fa29058d3690