SDK / exoplayer

Once we receive an update from a masked source, we first start the
preparation of an already pending period, and only then notify the
player of the new timeline. If the period prepares immediately inline,
the MediaPeriod.onPrepared callback arrives before the
onPlaylistUpdateRequested call in the Player. THis is the wrong order
and causes issues when the player tries to lookup information in the
timeline that doesn't exist yet.

This change fixes preroll playbacks before live streams.

PiperOrigin-RevId: 293340031