Bug 962655 - Use build-time preference to enable rocketbar

https://bugzilla.mozilla.org/show_bug.cgi?id=962655

* At build time, changes browser manifest to be role="system". This hides the browser icon in the UI.
* At build time, implements the view url activity in the system manifest so URLs will launch in the browser chrome.
* Sets rocketbar.enabled to true, unless ROCKETBAR=0 is specified.