Rename `EMPTY_MEDIA_ITEM` to `PLACEHOLDER_MEDIA_ITEM`

The `MediaItem` instances in the following cases are not actually empty but acts as a placeholder. `EMPTY_MEDIA_ITEM` can also be confused with `MediaItem.EMPTY`.

PiperOrigin-RevId: 495843012
(cherry picked from commit 3e7f53fd)

libraries/common/src/main/java/androidx/media3/common/Timeline.java

@@ -158,7 +158,7 @@ public abstract class Timeline implements Bundleable {
 
     private static final Object FAKE_WINDOW_UID = new Object();
 
-    private static final MediaItem EMPTY_MEDIA_ITEM =
+    private static final MediaItem PLACEHOLDER_MEDIA_ITEM =
         new MediaItem.Builder()
             .setMediaId("androidx.media3.common.Timeline")
             .setUri(Uri.EMPTY)
@@ -258,7 +258,7 @@ public abstract class Timeline implements Bundleable {
     /** Creates window. */
     public Window() {
       uid = SINGLE_WINDOW_UID;
-      mediaItem = EMPTY_MEDIA_ITEM;
+      mediaItem = PLACEHOLDER_MEDIA_ITEM;
     }
 
     /** Sets the data held by this window. */
@@ -281,7 +281,7 @@ public abstract class Timeline implements Bundleable {
         int lastPeriodIndex,
         long positionInFirstPeriodUs) {
       this.uid = uid;
-      this.mediaItem = mediaItem != null ? mediaItem : EMPTY_MEDIA_ITEM;
+      this.mediaItem = mediaItem != null ? mediaItem : PLACEHOLDER_MEDIA_ITEM;
       this.tag =
           mediaItem != null && mediaItem.localConfiguration != null
               ? mediaItem.localConfiguration.tag

libraries/exoplayer/src/main/java/androidx/media3/exoplayer/source/ConcatenatingMediaSource.java

@@ -61,7 +61,7 @@ public final class ConcatenatingMediaSource extends CompositeMediaSource<MediaSo
   private static final int MSG_UPDATE_TIMELINE = 4;
   private static final int MSG_ON_COMPLETION = 5;
 
-  private static final MediaItem EMPTY_MEDIA_ITEM =
+  private static final MediaItem PLACEHOLDER_MEDIA_ITEM =
       new MediaItem.Builder().setUri(Uri.EMPTY).build();
 
   // Accessed on any thread.
@@ -451,7 +451,7 @@ public final class ConcatenatingMediaSource extends CompositeMediaSource<MediaSo
   public MediaItem getMediaItem() {
     // This method is actually never called because getInitialTimeline is implemented and hence the
     // MaskingMediaSource does not need to create a placeholder timeline for this media source.
-    return EMPTY_MEDIA_ITEM;
+    return PLACEHOLDER_MEDIA_ITEM;
   }
 
   @Override
@@ -1018,7 +1018,7 @@ public final class ConcatenatingMediaSource extends CompositeMediaSource<MediaSo
 
     @Override
     public MediaItem getMediaItem() {
-      return EMPTY_MEDIA_ITEM;
+      return PLACEHOLDER_MEDIA_ITEM;
     }
 
     @Override

libraries/exoplayer/src/main/java/androidx/media3/exoplayer/source/MergingMediaSource.java

@@ -72,7 +72,7 @@ public final class MergingMediaSource extends CompositeMediaSource<Integer> {
   }
 
   private static final int PERIOD_COUNT_UNSET = -1;
-  private static final MediaItem EMPTY_MEDIA_ITEM =
+  private static final MediaItem PLACEHOLDER_MEDIA_ITEM =
       new MediaItem.Builder().setMediaId("MergingMediaSource").build();
 
   private final boolean adjustPeriodTimeOffsets;
@@ -163,7 +163,7 @@ public final class MergingMediaSource extends CompositeMediaSource<Integer> {
 
   @Override
   public MediaItem getMediaItem() {
-    return mediaSources.length > 0 ? mediaSources[0].getMediaItem() : EMPTY_MEDIA_ITEM;
+    return mediaSources.length > 0 ? mediaSources[0].getMediaItem() : PLACEHOLDER_MEDIA_ITEM;
   }
 
   @Override