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`.
You close dialogs by setting `.opened=false`: https://www.polymer-project.org/docs/elements/core-elements.html#core-overlay.