This change fixes two bugs where MediaSessionServe shows a notification
with the Play icon but tapping it will not start playback:
1. After playback ends: we need to seek to the beginning of the media
item.
2. After adding media items to the player but not starting playback:
We need to call Player.prepare() too.
PiperOrigin-RevId: 432469953
Where this introduced an inconsistency (e.g. assigning to something
called `windowIndex`), I generally renamed the transitive closure of
identifiers to maintain consistency (meaning this change is quite
large). The exception is code that interacts with Timeline and Window
directly, where sometimes I kept the 'window' nomenclature.
#minor-release
PiperOrigin-RevId: 407040052
