AtkListener shouldn't be holding a reference back to the context /
broadcaster it listens to, as the latter also holds a reference to
the former.

Change-Id: Ie75cc4667f614752db710c20acbb83b93783654f
Reviewed-on: https://gerrit.libreoffice.org/31063Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Kohei Yoshida <libreoffice@kohei.us>