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.

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