* Add iOS template

* Android

* Let the engine reset the theme without the activity knowing

* Small tweak

* Replace assets with different vectors

* Let the template hookup have no actual image assets

* Add back placeholder assets with 1px transparent pngs

* Fix drawable xml

* clean up an extraneous line in the storyboard xml

We want to use the FlutterViewController in this new project template.

Fixes #4840

This patch changes the default `flutter create` command to generate a
HelloServices-style app on iOS. As a consequence, the Atom integration now
fully works with HelloServices.