Add back deprecated getWindow to ForwardingTimeline.

Implementations of ForwardingTimeline may override any of the two variants of
this method. We need to ensure that the customized override is always called.

Add back the deprecated method and make it final to forward to the
non-deprecated method in all cases for ForwardingTimelines.

PiperOrigin-RevId: 265419830

library/core/src/main/java/com/google/android/exoplayer2/source/ForwardingTimeline.java

@@ -62,6 +62,12 @@ public abstract class ForwardingTimeline extends Timeline {
   }
 
   @Override
+  public final Window getWindow(
+      int windowIndex, Window window, boolean setIds, long defaultPositionProjectionUs) {
+    return getWindow(windowIndex, window, defaultPositionProjectionUs);
+  }
+
+  @Override
   public int getPeriodCount() {
     return timeline.getPeriodCount();
   }