Pref service: add a factory to create the client lib

services/preferences/manifest.json

 {
   "name": "preferences2",
   "display_name": "Preferences",
   "interface_provider_specs": {
     "service_manager:connector": {
       "provides": {
         "pref_client": [
+          "prefs::mojom::PrefStoreConnector",
           "prefs::mojom::PrefStoreRegistry",
           "prefs::mojom::PersistentPrefStoreConnector"
         ],
         "pref_control": [
           "prefs::mojom::PrefServiceControl"
         ]
       },
       "requires": {
       }
     }
   }
 }