Limit the TCCL when instantiating UntrustedXML
JAXP newInstance() methods can end up going through ServiceLoader
with TCCL as its ClassLoader, which means our constants may end up
pointing to a ClassLoader beyond our normal visibility.
This means we can end up picking up a different factory depending
on where we are called from and also can mean the TCCL cannot be
garbage-collected.
Force the TCCL to be the ClassLoader holding UntrustedXML, thus
establishing tight coupling between JAXP implementation discovery
mechanism and UntrustedXML.
JIRA: CONTROLLER-1867
Change-Id: Icacb0e193c7360c73bd07bc5c815bf803ca9ac44
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>