SDK / exoplayer

This change makes sure played server side ads are skipped in a single period
timeline. It avoids creating an ad-MediaPeriodInfo for played postrolls and
creates a content info instead. It also sets the end position for content infos
that terminate the stream before the stream is actually finished. This prevents
the player from continue playing the remaining media delivered by the
MediaPeriod.

We also make sure that the discontinuity of played ads are not reported because
there is actually no discontinuity.

#minor-release

PiperOrigin-RevId: 428734387