This commit turns on adaptive motion search for ARF coding, in
addition to other normal inter frame coding. It improves the
average compression efficiency:

stdhd 0.1%
derf  0.04%

For the test sequences, the speed 3 runtime is reduced:

pedestrian 1080p 2000 kbps, 149932 ms -> 144580 ms, (3.3% speed-up)
bus CIF 1000 kbps, 8050 ms -> 7895 ms, (1.9%)
highway CIF 100 bkps, 45033 ms -> 44078 ms, (2.2%)

Change-Id: I5228565b609f99e8ae04f6140a2bf2b64a831d21