This does not change the precision used for od_ec_enc_bool_q15
(used by aom_write and aom_write_bit), which means these two
paths will use different CDF precision for now.

The test that verifies that q15's code identically to CDFs
is disabled.

Change-Id: I4cd40a60d472eb58f37ac4fda1056b5dfe8b39a5