[Java] Add a reachabilityFence on underlying trait impl calls

Because the implementation instance is implicitly held as a
reference in the `held` field, it seems unlikely this could ever
matter, hwoever, in the simplified implementation in `PeerTest`
(where there is no Human instance to hold a reference to the
underlying bindings instance), we do see the implementation being
free'd while its still in use. Thus, to ensure we can't hit any
such issue in the future, we add one in normal human objects as
well.