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
(cherry picked from commit 1023b9d5)
