flip the flutter create language pref defaults to swift and kotlin

<img width="809" alt="Screen Shot 2019-08-13 at 1 28 50 PM" src="https://user-images.githubusercontent.com/1269969/62975042-547e8080-bdce-11e9-8f11-02326d71c717.png">

See https://github.com/flutter/flutter/pull/38339 for context - this is to match the new Flutter SDK defaults.