Comment 8 for bug 23369

Yes, I can use nsIObserver. But I think nsISystemPrefListener is simpler, only
one string to pass as parameter.

nsIPrefBranch is too large, most things in it is useless here. furthermore,
nsISystemPrefService also include the stuff that is in nsIPrefBranchInternal.
nsISystemPrefService can be replaced by "nsIPrefBranch + nsIPrefBranchInternal"
with many things left unimplemented, why not create a simple
nsISystemPrefService instead, and it also has a good name for that purpose.

>> Why does nsSystemPref::ReadSystemPref unconditionally lock the pref?
That is because I think all the preference coming from system has higher
priority and should be readonly, if user chose to read them from the system.