DERBY-6717: Policies with multiple SystemPermissions are not handled well

Override newPermissionCollection() and return an instance that is able
to distinguish between SystemPermissions with the same name and
different actions. The default implementation in BasicPermission only
considers the name of the permission.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1621257 13f79535-47bb-0310-9956-ffa450edef68