Clean up

app/src/main/java/com/hippo/scene/SceneApplication.java

@@ -21,7 +21,7 @@ import android.util.SparseArray;
 
 import com.hippo.yorozuya.IntIdGenerator;
 
-public class SceneApplication extends Application {
+public abstract class SceneApplication extends Application {
 
     private final IntIdGenerator mIdGenerator = new IntIdGenerator();
     private final SparseArray<StageActivity> mStageMap = new SparseArray<>();