DERBY-6629: Restrict privileged operation in CreateXMLFile

Remove the privileged operation in the CreateXMLFile class. Instead, let
the caller do the privileged operation and pass in a Writer object that
CreateXMLFile can write the file to.

Make the tests invoke the plan exporter tool via its main class instead
of calling the internal API.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1605021 13f79535-47bb-0310-9956-ffa450edef68