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'd probably do these cleanup tasks in a handler for the [`core-overlay-close-completed`](https://www.polymer-project.org/docs/elements/core-elements.html#core-overlay) event, so that we're sure to catch every user action that can close the dialog. Each place that needs to close the dialog can just call the `$.deviceSelectorDialog.close()` method, and it'll eventually flow through to the event.