BUG-868: migrate ListenerRegistry

yangtools.concepts.util is being migrated to yangtools.util, so adjust
users to lower the number of warnings.

Change-Id: I652f0131a59fdb2481d779c181861f7ed7674b03
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-868: migrate ListenerRegistry"