Add explicit protected constructor to Timeline.

Timeline is already abstract, so it can only be constructed from a
subclass anyway.

PiperOrigin-RevId: 389827960

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

@@ -983,6 +983,8 @@ public abstract class Timeline implements Bundleable {
         }
       };
 
+  protected Timeline() {}
+
   /** Returns whether the timeline is empty. */
   public final boolean isEmpty() {
     return getWindowCount() == 0;