SubtitleView now becomes a ViewGroup that owns a SubtitleTextView. It
handles some common styling defaults, but delegates the underlying
values down into SubtitleTextView through the SubtitleView.Output
interface.
When I add a SubtitleWebView this will also be a ViewGroup containing
a WebView and will implement SubtitleView.Output and convert Cue styling
into HTML & CSS.
PiperOrigin-RevId: 291903294
