RFR (JAXP) 8170556: Warnings cleanup related to JDK-8167340

[Joe Wang]

Hi,

This was the cleanup portion of the change for JDK-8167340. As Lance 
suggested, it was split from the original webrev. In addition to that 
cleanup, I've added coverage to the entire StAX packages. This cleanup 
will reduce 138 warnings.

jbs: https://bugs.openjdk.java.net/browse/JDK-8170556
webrev: http://cr.openjdk.java.net/~joehw/jdk9/8170556/webrev/

Thanks,
Joe