Notify modalDismiss if no modals are displayed

Notify the Activity layout that the top most Modal was dismissed only
if no other modals are displayed. Currently this event is used as a
`onVisible` event to to the layout. This whole logic can be simplified
when we implement proper screen visibility events.

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

         destroy();
         onModalDismissedListener.onModalDismissed(this);
     }
+
+    public void onModalDismissed() {
+        layout.onModalDismissed();
+    }
 }

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

     @Override
     public void onModalDismissed(Modal modal) {
         stack.remove(modal);
+        if (isShowing()) {
+            stack.peek().onModalDismissed();
+        }
         EventBus.instance.post(new ModalDismissedEvent());
     }
 

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

     @Override
     public void onEvent(Event event) {
         if (event.getType().equals(ModalDismissedEvent.TYPE)) {
-            layout.onModalDismissed();
+            handleModalDismissedEvent();
         } else if (event.getType().equals(JsDevReloadEvent.TYPE)) {
-            modalController.destroy();
-            layout.destroy();
+            handleJsDevReloadEvent();
         }
     }
+
+    private void handleModalDismissedEvent() {
+        if (!modalController.isShowing()) {
+            layout.onModalDismissed();
+        }
+    }
+
+    private void handleJsDevReloadEvent() {
+        modalController.destroy();
+        layout.destroy();
+    }
 }