Move code for reading and writing literals and reading trees to use
just the aom_read_bit() and aom_write_bit() function calls.
Change-Id: Id2bced5f0125a5558030a813c51c3d79e5701873
(cherry picked from aom/master commit bc1ac15846)
This should make room for compile time pluggable replacements.
Change-Id: Ib7afcffa93bf664b89a49da21a20138127443292
(cherry picked from commit 9dd0b89824)