Allows override of DefaultSplashLayout by child class (#4084)

This is mentionned in https://github.com/wix/react-native-navigation/issues/3905

lib/android/app/src/main/java/com/reactnativenavigation/NavigationActivity.java

         navigator.destroyViews();
     }
 
-    private void addDefaultSplashLayout() {
+    protected void addDefaultSplashLayout() {
         View view = new View(this);
         view.setBackgroundColor(Color.WHITE);
         setContentView(view);