Enable adaptive motion search for ARF coding

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