DERBY-6741: Add a privilege barrier to prevent users from getting a ContextManager from an embedded connection object; tests passed cleanly on derby-6741-01-aa-usederbyinternals.diff.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1626274 13f79535-47bb-0310-9956-ffa450edef68