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`.
Let's define these outside of the `getDevices()` callback to be sure they're constant. Maybe in the `created` event for the `request-device-window` element. However, we should do that in a separate change, to make the diff for this change as easy to read as possible.