applet: ensure to create the secret agent instance only once

Since we don't disconnect shell_version_changed_cb(), make sure that
we don't call register_agent() when an agent is already registered.

https://bugzilla.gnome.org/show_bug.cgi?id=727923

Signed-off-by: Thomas Haller <thaller@redhat.com>
(cherry picked from commit 1731d6db5176f17f88fdcc48f99916101b8a12ae)