Bug 910552 - [Call Setting] Call Barring menu not implemented

To make the module more re-usable and testable, I would encourage not to include the logic regarding UI in it. That said, the module should only provide functions for manipulating or retrieving call barring settings. To be more specific, the call barring settings are five properties of an Observable (you could refer to the wallpaper module as an example[1]), and then we can wire the UI (currently check boxes) to the properties in `panel.js` which should responsible for UI logic.

[1]: https://github.com/mozilla-b2g/gaia/blob/master/apps/settings/js/panels/display/wallpaper.js