Mark BasePlayer constructor as protected

This is an abstract class so it can only be constructed from the context
of a subclass anyway.

PiperOrigin-RevId: 382756293

library/common/src/main/java/com/google/android/exoplayer2/BasePlayer.java

@@ -28,7 +28,7 @@ public abstract class BasePlayer implements Player {
 
   protected final Timeline.Window window;
 
-  public BasePlayer() {
+  protected BasePlayer() {
     window = new Timeline.Window();
   }