The motion vector search result at single reference frame mode is stored
for later use. There are some issues in current implementation.
1. single_newmv is only stored for 1st ref_mv, but not for other ref_mv
values tested.
2. If single reference mode is skipped and not tested, single_newmv is
not available.
3. Interintra mode always use single_newmv as its MV, which may not be
available as mentioned above.
To fix it, we store single_newmv for every ref_mv tested. In interintra
mode, if single_newmv is not available, do motion search to find a MV.

This was revealed by a motion vector unit test failure.

BUG=aomedia:1190

Change-Id: I3e690587644d4c8c31337f37380b796a3d71a3f6