Pref service: add a factory to create the client lib

services/BUILD.gn

Index: services/BUILD.gn
diff --git a/services/BUILD.gn b/services/BUILD.gn
index 31a46c33983ed4f421183a06e2cddf9ad3d247fc..4b81d47dd216d9be09d1856d9a058affb00438f3 100644
--- a/services/BUILD.gn
+++ b/services/BUILD.gn
@@ -17,6 +17,7 @@ service_test("service_unittests") {
   deps = [
     "//services/device:tests",
     "//services/image_decoder:tests",
+    "//services/preferences:tests",
     "//services/resource_coordinator:tests",
     "//services/shape_detection:tests",
   ]
@@ -35,5 +36,9 @@ service_test("service_unittests") {
 }
 
 catalog("service_unittests_catalog") {
-  catalog_deps = [ "//services/video_capture:tests_catalog" ]
+  testonly = true
+  catalog_deps = [
+    "//services/preferences:tests_catalog",
+    "//services/video_capture:tests_catalog",
+  ]
 }