Update comments on nonexistent vpx_codec_init

Update comments on the nonexistent vpx_codec_init() function. Replace it
with vpx_codec_dec_init() and vpx_codec_enc_init().

based on the change in libaom:
b1b8c68e8 Update comments on nonexistent aom_codec_init

Change-Id: I63d3f6c87706a98f631457b5f6ce51e8b0c5cfb1
2 files changed