In non-embedded firefox ios, amp-app-banner should be shown.
Visit https://amphtml-nightly.herokuapp.com/examples/article.amp.html on firefox ios

https://github.com/ampproject/amphtml/blob/master/extensions/amp-app-banner/0.1/amp-app-banner.js#L219
has wrong value for this.canShowBuiltinBanner_ because platform.isSafari() is wrong