Add OptIn annotation to method declaration in demo app file

#cherrypick

PiperOrigin-RevId: 648641357

demos/main/src/main/java/androidx/media3/demo/main/DemoUtil.java

@@ -100,6 +100,7 @@ public final class DemoUtil {
         .setExtensionRendererMode(extensionRendererMode);
   }
 
+  @OptIn(markerClass = androidx.media3.common.util.UnstableApi.class)
   public static synchronized DataSource.Factory getHttpDataSourceFactory(Context context) {
     if (httpDataSourceFactory != null) {
       return httpDataSourceFactory;