Use media source tag in dummy timeline.

This is now possible as it's directly accessible from the media source.

Issue:#5177
Issue:#5155
PiperOrigin-RevId: 224321917

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

@@ -826,7 +826,7 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
 
     public MediaSourceHolder(MediaSource mediaSource) {
       this.mediaSource = mediaSource;
-      this.timeline = new DeferredTimeline();
+      this.timeline = DeferredTimeline.createWithDummyTimeline(mediaSource.getTag());
       this.activeMediaPeriods = new ArrayList<>();
       this.uid = new Object();
     }
@@ -951,11 +951,19 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
   private static final class DeferredTimeline extends ForwardingTimeline {
 
     private static final Object DUMMY_ID = new Object();
-    private static final DummyTimeline DUMMY_TIMELINE = new DummyTimeline();
 
     private final Object replacedId;
 
     /**
+     * Returns an instance with a dummy timeline using the provided window tag.
+     *
+     * @param windowTag A window tag.
+     */
+    public static DeferredTimeline createWithDummyTimeline(@Nullable Object windowTag) {
+      return new DeferredTimeline(new DummyTimeline(windowTag), DUMMY_ID);
+    }
+
+    /**
      * Returns an instance with a real timeline, replacing the provided period ID with the already
      * assigned dummy period ID.
      *
@@ -968,11 +976,6 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
       return new DeferredTimeline(timeline, firstPeriodUid);
     }
 
-    /** Creates deferred timeline exposing a {@link DummyTimeline}. */
-    public DeferredTimeline() {
-      this(DUMMY_TIMELINE, DUMMY_ID);
-    }
-
     private DeferredTimeline(Timeline timeline, Object replacedId) {
       super(timeline);
       this.replacedId = replacedId;
@@ -1016,6 +1019,12 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
   /** Dummy placeholder timeline with one dynamic window with a period of indeterminate duration. */
   private static final class DummyTimeline extends Timeline {
 
+    @Nullable private final Object tag;
+
+    public DummyTimeline(@Nullable Object tag) {
+      this.tag = tag;
+    }
+
     @Override
     public int getWindowCount() {
       return 1;
@@ -1025,7 +1034,7 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
     public Window getWindow(
         int windowIndex, Window window, boolean setTag, long defaultPositionProjectionUs) {
       return window.set(
-          /* tag= */ null,
+          tag,
           /* presentationStartTimeMs= */ C.TIME_UNSET,
           /* windowStartTimeMs= */ C.TIME_UNSET,
           /* isSeekable= */ false,