Pref service: add a factory to create the client lib

services/preferences/unittest_manifest.json

Index: services/preferences/unittest_manifest.json
diff --git a/services/preferences/unittest_manifest.json b/services/preferences/unittest_manifest.json
new file mode 100644
index 0000000000000000000000000000000000000000..8c4e582762a3b4d2a44640f3ae124c9e6f0c1963
--- /dev/null
+++ b/services/preferences/unittest_manifest.json
@@ -0,0 +1,16 @@
+{
+  "name": "prefs_unittests",
+  "display_name": "Prefs Unittests",
+  "interface_provider_specs": {
+    "service_manager:connector": {
+      "provides": {
+        "service_manager:service_factory": [
+          "service_manager::mojom::ServiceFactory"
+        ]
+      },
+      "requires": {
+        "preferences2": [ "pref_client", "pref_control" ]
+      }
+    }
+  }
+}