This plumbs a duration through `SampleQueue` which will make it easier
to handle transcoding muxed subtitles from e.g. SubRip to
`text/x-exoplayer-cues`.
This change is a no-op to the end-to-end behaviour of ExoPlayer because
currently we only support parsing sideloaded subtitles before
`SampleQueue`, and by adding the duration we don't affect the cues that
are ultimately output by `Player.Listener.onCues` (as shown by no change
to the golden files for `WebvttPlaybackTest` in this commit).
I considered making `CuesWithStartTimeAndDuration` implement
`Bundleable` (and deleting `CueEncoder/Decoder`) but decided against
it because we are deliberately not encoding `startTimeUs` (since that's
encoded as the sample time in `SampleQueue`). I also considered
introducing another type that only has `List<Cue>` and `durationUs`
fields, but it didn't seem necessary, since we want `startTimeUs`
everywhere else (except inside `SampleQueue`).
PiperOrigin-RevId: 545226847
a783d704b2