Merge Cea708Parser back into Cea708Decoder

This reverses 94e45eb

Due to the re-ordering of packets done in `CeaDecoder`, there's no way
to use the current implementation to correctly parse these subtitle
formats during extraction (the `SubtitleParser` interface), so we have
to keep the `SubtitleDecoder` implementations.

#minor-release

PiperOrigin-RevId: 604350951
(cherry picked from commit 51b4fa2)

Author: icbaker