Make `Rating.RATING_UNSET` package-private

This value is only needed by subclasses of `Rating`, all of which are
in this package (the `Rating` constructor is already package-private to
ensure this).

PiperOrigin-RevId: 450886872

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

@@ -31,7 +31,7 @@ import java.lang.annotation.Target;
 public abstract class Rating implements Bundleable {
 
   /** A float value that denotes the rating is unset. */
-  public static final float RATING_UNSET = -1.0f;
+  /* package */ static final float RATING_UNSET = -1.0f;
 
   // Default package-private constructor to prevent extending Rating class outside this package.
   /* package */ Rating() {}