vp9: Use sb content measure to bias against golden.

vp9: Use sb content measure to bias against golden.

For each superblock, keep track of how far from current frame
was the last significant content change, and use that (along
with GF distance), to turnoff GF search in non-rd pickmode.

Only enabled for speed >= 8.

avgPNSR on RTC/RTC_derf down by ~0.9/1.2.
Speedup on mac: ~3-5%.
Speedup on arm: 3.6% for VGA and 4.4% for HD.

Change-Id: Ic3f3d6a2af650aca6ba0064d2b1db8d48c035ac7