Consolidate prefs usage in torbutton.js
In torbutton.js, we call
var pref_service = Components.classes["@mozilla.org/preferences-service;1"]
.getService(Components.interfaces.nsIPrefBranch);
or
m_tb_prefs = Components.classes["@mozilla.org/preferences-service;1"]
.getService(Components.interfaces.nsIPrefBranch);
7 times when that functionality is already available in Services.prefs
. So I am proposing to consolidate this.