Refactor SEP prepare to clarify that it is equivalent to EPI prepare.

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

library/core/src/main/java/com/google/android/exoplayer2/SimpleExoPlayer.java

@@ -1354,10 +1354,7 @@ public class SimpleExoPlayer extends BasePlayer
   @Override
   public void prepare(MediaSource mediaSource, boolean resetPosition, boolean resetState) {
     verifyApplicationThread();
-    setMediaSources(
-        Collections.singletonList(mediaSource),
-        /* startWindowIndex= */ resetPosition ? 0 : C.INDEX_UNSET,
-        /* startPositionMs= */ C.TIME_UNSET);
+    setMediaSources(Collections.singletonList(mediaSource), resetPosition);
     prepare();
   }