This commit makes the decoder to construct the reference motion
vector candidate list only for the selected reference frames. This
improves the decoder speed. For the test clip pedestrian_area at
1080p, coded at 3000 kbps, the decoding speed goes up from 58.4 fps
to 72 fps.

Change-Id: I257c521f46221936028ae01f205520d94ba7f16e