Expose layout in NavigationActivity (#4043)

Use Case: When exposing the layout, other android code can check what kind of layout it is and react accordingly, fx:
```
if (activity.getLayout() instanceof BottomTabsLayout) {
    // ...
}
```

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

     public static void setStartAppPromise(Promise promise) {
         NavigationActivity.startAppPromise = promise;
     }
+
+    public Layout getLayout() {
+        return layout;
+    }
 }