[prefs] Use the default prefservice CID
[prefs] Use the default prefservice CID

Use the CID for the pref service defined in the Mozilla headers. This
shouldn't be necessary, but there's a possibility that it could be
required somewhere in the code, or possibly in the future, so use it
just to be sure.

clutter-project/clutter-mozembed,clutter-project/clutter-mozembed,clutter-project/clutter-mozembed