When the start position of a MediaPeriodInfo is equal or higher than the duration,
we set the start position to `duration - 1` to end on the last frame. With server
side inserted ad streams, this has the effect that we actually need to seek back to
the last content frame after a post-roll.
This is desirable when actually ending on that frame but produces a BUFFERING event
when transitioning from an SSAI stream with a post-roll to the next media item in
the playlist. This change sets the start position to the duration when we are
clipping the last content period of an SSAI stream that is played in a playlist.
PiperOrigin-RevId: 433445680
e5d1466731