feat(MediaSource): provide getTag default implementation

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

@@ -222,7 +222,9 @@ public interface MediaSource {
   /**
    * Returns the tag set on the media source, or null when none was set.
    */
-  @Nullable Object getTag();
+  @Nullable default Object getTag() {
+    return null;
+  }
 
   /**
    * Starts source preparation if not yet started, and adds a listener for timeline and/or manifest