Before this change:
- SimpleExoPlayer.prepare(mediaSource) ended up calling
ExoPlayerImpl.setMediaSourcesInternal() with startWindowIndex=0 and
resetToDefaultPosition=false.
- ExoPlayerImpl.prepare(mediaSource) ended up calling
ExoPlayerImpl.setMediaSourcesInternal() with
startWindowIndex=C.INDEX_UNSET and resetToDefaultPosition=true.
This was functionaly equivalent but a bit confusing.
#minor-release
PiperOrigin-RevId: 368818143
