Enable checkstyle rule to hide utility class constructors.

Enabled "HideUtilityClassConstructor" checkstyle rule and
fixed the violations that were exposed.

Change-Id: I66ae4c197941af6a39e8ddda4e0175502be69c7e
diff --git a/conf/checkstyle/sun_checks.xml b/conf/checkstyle/sun_checks.xml
index 3d99e59..f695fc7 100644
--- a/conf/checkstyle/sun_checks.xml
+++ b/conf/checkstyle/sun_checks.xml
@@ -240,9 +240,7 @@
         <!-- <module name="DesignForExtension"/> -->
         <module name="FinalClass"/>
 
-        <module name="HideUtilityClassConstructor">
-          <property name="severity" value="warning"/>
-        </module>
+        <module name="HideUtilityClassConstructor"/>
 
         <module name="InterfaceIsType"/>