Mark FakeMediaSourceFactory final

There's no need to extend this class. Factories for subclasses of
FakeMediaSource will need to re-implement createMediaSource, at which
point they basically need to re-implement the whole factory interface.

PiperOrigin-RevId: 417817499

libraries/test_utils/src/main/java/androidx/media3/test/utils/FakeMediaSourceFactory.java

@@ -29,7 +29,7 @@ import androidx.media3.test.utils.FakeTimeline.TimelineWindowDefinition;
 
 /** Fake {@link MediaSourceFactory} that creates a {@link FakeMediaSource}. */
 @UnstableApi
-public class FakeMediaSourceFactory implements MediaSourceFactory {
+public final class FakeMediaSourceFactory implements MediaSourceFactory {
 
   /** The window UID used by media sources that are created by the factory. */
   public static final Object DEFAULT_WINDOW_UID = new Object();