SDK / exoplayer

MaskingMediaSource needs to resolve the prepare position set for a MaskingPeriod
while the source was still unprepared to the first actual prepare position.

It currently assumes that the period-window offset and the default position is
zero. This assumption is correct when a PlaceholderTimeline is used, but it
may not be true if the real timeline is already known (e.g. when re-preparing
a live stream after a playback error).

Fix this by using the known timeline at the time of the preparation.
Also:
 - Update a test that should have caught this to use lazy re-preparation.
 - Change the demo app code to use the recommended way to restart playback
   after a BehindLiveWindowException.

Issue: #8675
PiperOrigin-RevId: 361604191