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

testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeMediaSourceFactory.java

@@ -27,7 +27,7 @@ import com.google.android.exoplayer2.upstream.LoadErrorHandlingPolicy;
 import com.google.android.exoplayer2.util.Util;
 
 /** Fake {@link MediaSourceFactory} that creates a {@link FakeMediaSource}. */
-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();