Bug 913326 - Set voice privacy mode at startup r=alive, jaoo

https://bugzilla.mozilla.org/show_bug.cgi?id=913326

- Add a helper maintaining the settings value. We can use it to get the current voice privacy settings.
- For all call related settings (for example roaming preference in the upcoming patch) we will put them in system/js/call_settings.js so that we can restore user settings at startup.