Return concrete SurfaceAssetLoader from factory

If the caller is another asset loader (or another asset loader factory) that is
delegating to a `SurfaceAssetLoader`, it may want to call methods on the
`SurfaceAssetLoader`. Return the concrete type so that a cast isn't necessary
at the call site.

PiperOrigin-RevId: 712881130

libraries/transformer/src/main/java/androidx/media3/transformer/SurfaceAssetLoader.java

@@ -87,7 +87,7 @@ public final class SurfaceAssetLoader implements AssetLoader {
     }
 
     @Override
-    public AssetLoader createAssetLoader(
+    public SurfaceAssetLoader createAssetLoader(
         EditedMediaItem editedMediaItem,
         Looper looper,
         AssetLoader.Listener listener,