This change uses the new incremental overloads of `SubtitleParser` to
avoid introducing the performance regression caused by the original
change requiring all cues to be fully parsed before the first could be
shown on screen.
`TtmlDecoder` which used to be `SimpleSubtitleDecoder` will now be
called `TtmlParser` and implement `SubtitleParser` interface. For
backwards compatibility, we will have the same functionality provided
by `DelegatingSubtitleDecoder` backed-up by a new `TtmlParser`
instance.
PiperOrigin-RevId: 566671398
694d040d7e