Current encoding of single ref assigns bit 0 to LAST_FRAME and bit 1
to LAST2_frame, whereas the encoding of compound ref assigns bit 1 to
LAST_FRAME and bit 0 to LAST_FRAME. This patch unifies the design and
makes the binary symbol assignment for LAST/LAST2 consistent
regardless of the single/compound ref scenarios.

This patch incurs a small syntax change but should not have noticeable
coding performance impact. Once BUG 973 is fully addressed, default
cdfs for reference frame coding will be updated.

BUG=aomedia:973

Change-Id: I1f3027965c0bb2d343bc8ad873f7c0015123f151