Add a new experimental flag SBL_SYMBOL

A new experiment SBL_SYMBOL, meaning superblock-level symbols, will
be explored. It allows some symbols being coded at superblock level
(64x64) by checking whether a symbol(e.g. motion_mode, tx depth,
and interpolation filter) is identical across macroblocks in a
superblock.

Change-Id: I38408325c9b7a4b94c11c400a5060036ce36405e