Switch to using ListenerRegistration from yangtools

Yangtools concepts have been updated to mmatch our ListenerRegistration
semantics, so there's no reason to keep it around. Migrate our code and
remove it from our concepts.

Also remove Immutable interface. It has been deprecated and unused --
again, yangtools concepts carry the equivalent interface.

Change-Id: I28e5b4e06df0e0b39f808768763c341ad166e352
Signed-off-by: Robert Varga <rovarga@cisco.com>