Admin{Constants|Messages}: Move static instances from Util class

The Util class will eventually be removed after the project access
RPC is replaced with the REST API.

As a preparatory step, move the constants and messages instances out
of this class into the respective interfaces. This also makes these
implementations consistent with how messages and constants are defined
in other such interfaces.

Change-Id: Ie9fbfb901870d7ce7caad5cbb253194f81632c0f

gerrit-gwtui/src/main/java/com/google/gerrit/client/admin/AdminConstants.java

@@ -14,10 +14,13 @@
 
 package com.google.gerrit.client.admin;
 
+import com.google.gwt.core.client.GWT;
 import com.google.gwt.i18n.client.Constants;
 import java.util.Map;
 
 public interface AdminConstants extends Constants {
+  AdminConstants I = GWT.create(AdminConstants.class);
+
   String defaultAccountName();
 
   String defaultAccountGroupName();