Add Checkstyle StringLiteralEquality

Author: garydgregory

src/conf/checkstyle.xml

@@ -71,6 +71,7 @@ limitations under the License.
     </module>
     <module name="SimplifyBooleanExpression" />
     <module name="SimplifyBooleanReturn" />
+    <module name="StringLiteralEquality" />
     <module name="TypecastParenPad" />
     <module name="UnusedImports" />
     <module name="WhitespaceAfter" />