Dismiss all modals before app start to ensure this use case does not break (#6152)

This commit adds a call to dismissAllModals before setRoot to ensure this use case doesn't break in the future.

playground/src/app.js

@@ -22,6 +22,7 @@ function start() {
   registerScreens();
   Navigation.events().registerAppLaunchedListener(async () => {
     setDefaultOptions();
+    Navigation.dismissAllModals();
     setRoot();
   });
 }