Calling aom_write_bit() and aom_read_bit() with --enable-daala_ec
 would call aom_write() and aom_read() with probability 128 which
 would ultimately call od_ec_enc_bits() and od_ec_dec_bits().
This refactors that code and makes the call explicit.

objective-1-fast:
master@2016-12-14T18:38:33Z -> daala_ec_bits@2016-12-14T18:36:22Z

    PSNR | PSNR Cb | PSNR Cr | PSNR HVS |   SSIM | MS SSIM | CIEDE 2000
  0.0000 |  0.0000 |  0.0000 |   0.0000 | 0.0000 |  0.0000 |     0.0000

Change-Id: Ib69e98734fadcdc8b89936b7b6fbd0574afc7e34