Make ExoPlayerImpl an ExoPlayer implementation

All the functionality of SimpleExoPlayer has moved to ExoPlayerImpl.
Hence, ExoPlayerImpl can fulfil its own name and become an ExoPlayer
implementation. As a result, ExoPlayer.Builder can return ExoPlayerImpl
directly without using SimpleExoPlayer at all.

#minor-release

PiperOrigin-RevId: 427947028

libraries/exoplayer/src/main/java/androidx/media3/exoplayer/ExoPlayer.java

@@ -1018,7 +1018,9 @@ public interface ExoPlayer extends Player {
      * @throws IllegalStateException If this method has already been called.
      */
     public ExoPlayer build() {
-      return buildSimpleExoPlayer();
+      checkState(!buildCalled);
+      buildCalled = true;
+      return new ExoPlayerImpl(/* builder= */ this, /* wrappingPlayer= */ null);
     }
 
     /* package */ SimpleExoPlayer buildSimpleExoPlayer() {