In a period transition we pass the start time of the next period, for
the final period we pass the duration of the period or timeline, if
known.
This means sideloaded subtitles now respect the end point of
ClippingMediaSource and ensures that content subtitles aren't
incorrectly displayed over mid-roll ads.
When transitioning back into the subtitled content the subtitles still
appear slightly before the video transitions, meaning the first subtitle
of the content is shown with the last few frames of the ad. Resolving
this in a way that doesn't break anything else requires a deeper
investigation.
Issue: #5317
Issue: #8456
#minor-release
PiperOrigin-RevId: 361797118
ff8db9a4fe