Make ExoPlayer.Builder#build return SimpleExoPlayer

In a future change it will be updated to return ExoPlayer

We no longer need separate methods to build Player and ExoPlayer, so
buildExoPlayer will be removed shortly.

PiperOrigin-RevId: 401441016

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

@@ -838,12 +838,12 @@ public interface ExoPlayer extends Player {
     }
 
     /**
-     * Builds a {@link Player} instance.
+     * Builds a {@link SimpleExoPlayer} instance.
      *
      * @throws IllegalStateException If this method or {@link #buildExoPlayer()} has already been
      *     called.
      */
-    public Player build() {
+    public SimpleExoPlayer build() {
       return buildExoPlayer();
     }