Use a dialog view instead of creating a new window

`chrome.app.window.create` can not be used to create more than one window in [mobile-chrome-apps](https://github.com/MobileChromeApps/mobile-chrome-apps).

This changed request device window to a dialog. This will also require to add `<web-bluetooth></web-bluetooth>` to the end of html body after importing `bluetooth.html`.
I'm nervous about adding a generically-named element to any page that uses this polyfill. Maybe name it "`web-bluetooth-request-device-dialog`"? ("dialog" instead of "window" since that's what it is now.)