Check if modal contains navigator

When ivoking navigation actions such as push, pop etc, instead of
checking modal visibility, check if modal actually contains the given
navigator. This fix alows to change navigation state in screens while
modal is displayed.

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

         layout.newStack(params);
     }
 
+    public boolean containsNavigator(String navigatorId) {
+        return layout.containsNavigator(navigatorId);
+    }
+
     @Override
     public void destroy() {
         layout.destroy();

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

         this.activity = activity;
     }
 
+    public boolean containsNavigator(String navigatorId) {
+        for (Modal modal : stack) {
+            if (modal.containsNavigator(navigatorId)) {
+                return true;
+            }
+        }
+        return false;
+    }
+
     public void showModal(ScreenParams screenParams) {
         Modal modal = new Modal(activity, this, screenParams);
         modal.show();

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

     }
 
     void push(ScreenParams params) {
-        if (modalController.isShowing()) {
+        if (modalController.containsNavigator(params.getNavigatorId())) {
             modalController.push(params);
         } else {
             layout.push(params);
     }
 
     void pop(ScreenParams params) {
-        if (modalController.isShowing()) {
+        if (modalController.containsNavigator(params.getNavigatorId())) {
             modalController.pop(params);
         } else {
             layout.pop(params);
     }
 
     void popToRoot(ScreenParams params) {
-        if (modalController.isShowing()) {
+        if (modalController.containsNavigator(params.getNavigatorId())) {
             modalController.popToRoot(params);
         } else {
             layout.popToRoot(params);
     }
 
     void newStack(ScreenParams params) {
-        if (modalController.isShowing()) {
+        if (modalController.containsNavigator(params.getNavigatorId())) {
             modalController.newStack(params);
         } else {
             layout.newStack(params);

android/app/src/main/java/com/reactnativenavigation/layouts/BottomTabsLayout.java

         EventBus.instance.post(new ScreenChangedEvent(getCurrentScreenStack().peek().getScreenParams()));
     }
 
+    @Override
+    public boolean containsNavigator(String navigatorId) {
+        // Unused
+        return false;
+    }
+
     public void selectBottomTabByTabIndex(Integer index) {
         bottomTabs.setCurrentItem(index);
     }

android/app/src/main/java/com/reactnativenavigation/layouts/Layout.java

     void showSnackbar(SnackbarParams params);
 
     void onModalDismissed();
+
+    boolean containsNavigator(String navigatorId);
 }

android/app/src/main/java/com/reactnativenavigation/layouts/SingleScreenLayout.java

         EventBus.instance.post(new ScreenChangedEvent(stack.peek().getScreenParams()));
     }
 
+    @Override
+    public boolean containsNavigator(String navigatorId) {
+        return stack.getNavigatorId().equals(navigatorId);
+    }
+
     @Override
     public boolean onTitleBarBackButtonClick() {
         if (leftButtonOnClickListener != null) {