SDK / exoplayer

This change makes sure that the `AdPlaybackState` of any period can
contain an empty postroll placeholder.

The placeholder postroll should be represented in the `MediaPeriodId`
of a content period as `nextAdGroupIndex`, but should be ignored when
building the list of `MediaPeriodInfo` in the `MediaPeriodQueue`. This
is required to allow to add an ad group to ad playback state of the
content period that is currently being played, instantly insert an ad
period into the media period queue and immediately transition playback
to the new period.

This change makes sure and tests that

- a live server side inserted postroll placeholder can be inserted to
  a `AdPlaybackState` in well-defined and tested way (helper method)
- a postroll placeholder is NOT ignored when
  `AdPlaybackState.getAdGroupIndexAfterPositionUs` is called (this
   is required when evaluating the `nextAdGroupIndex`).
- a postroll placeholder is ignored when
  `AdPlaybackState.getAdGroupIndexForPositionUs` is called (this is
  required to not attempt to play the ad and is analogous to ignore the
  post roll placeholder in a single period timeline).
- `MediaPeriod.getFollowingMediaPeriodInfo()` does not include a
  `MediaPeriodInfo` for the placeholder postroll when building the
   queue.

PiperOrigin-RevId: 515079136