Show in app notifications on modals (#866)

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

 import com.reactnativenavigation.params.FabParams;
 import com.reactnativenavigation.params.Orientation;
 import com.reactnativenavigation.params.ScreenParams;
+import com.reactnativenavigation.params.SlidingOverlayParams;
 import com.reactnativenavigation.params.TitleBarButtonParams;
 import com.reactnativenavigation.params.TitleBarLeftButtonParams;
 
         layout.dismissContextualMenu(screenInstanceId);
     }
 
+    void showSlidingOverlay(SlidingOverlayParams params) {
+        layout.showSlidingOverlay(params);
+    }
+
+    void hideSlidingOverlay() {
+        layout.hideSlidingOverlay();
+    }
+
     @Override
     public boolean onTitleBarBackButtonClick() {
         if (!layout.onBackPressed()) {

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

 import com.reactnativenavigation.params.ContextualMenuParams;
 import com.reactnativenavigation.params.FabParams;
 import com.reactnativenavigation.params.ScreenParams;
+import com.reactnativenavigation.params.SlidingOverlayParams;
 import com.reactnativenavigation.params.TitleBarButtonParams;
 import com.reactnativenavigation.params.TitleBarLeftButtonParams;
 
     public void onSideMenuButtonClick() {
         // Do nothing and let the layout handle the click
     }
+
+    void showSlidingOverlay(SlidingOverlayParams params) {
+        stack.peek().showSlidingOverlay(params);
+    }
+
+    void hideSlidingOverlay() {
+        stack.peek().hideSlidingOverlay();
+    }
 }

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

     }
 
     public void showSlidingOverlay(SlidingOverlayParams params) {
-        layout.showSlidingOverlay(params);
+        if (modalController.isShowing()) {
+            modalController.showSlidingOverlay(params);
+        } else {
+            layout.showSlidingOverlay(params);
+        }
     }
 
     public void hideSlidingOverlay() {
-        layout.hideSlidingOverlay();
+        if (modalController.isShowing()) {
+            modalController.hideSlidingOverlay();
+        } else {
+            layout.hideSlidingOverlay();
+        }
     }
 
     public void showSnackbar(SnackbarParams params) {