Reuse AbstractListenerRegistration

No sense in defining a new class -- just reuse what's already there.
Also uncovers the fact that we're losing an underlying registration --
but let's deal with that once the ListenerRegistration API changes not
to throw exceptions on close().

Change-Id: I5a12b1d7ca89f1608bc54b3f1a520223e650d53f
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Reuse AbstractListenerRegistration"