experiment with CONTEXT conversion

This commit changed the ENTROPY_CONTEXT conversion between MBs that
have different transform sizes.

In additioin, this commit also did a number of cleanup/bug fix:
1. removed duplicate function vp9_fix_contexts() and changed to use
vp8_reset_mb_token_contexts() for both encoder and decoder
2. fixed a bug in stuff_mb_16x16 where wrong context was used for
the UV.
3. changed reset all context to 0 if a MB is skipped to simplify the
logic.

Change-Id: I7bc57a5fb6dbf1f85eac1543daaeb3a61633275c