OF-774: Restore private constructor

Per review feedback, restore the private constructor for this static
utility class.

src/java/org/jivesoftware/openfire/auth/AuthorizationManager.java

@@ -116,6 +116,9 @@ public class AuthorizationManager {
         }
     }
 
+    // static utility class; do not instantiate
+    private AuthorizationManager() { }
+
     /**
      * Returns the currently-installed AuthorizationProvider. Warning: You
      * should not be calling the AuthorizationProvider directly to perform