manager: update session-id property when reusing display

If a display starts out its life as a greeter display, and then
gets reused for the user session, we need to update the session-id
property on the display to match its new session.

This is important so the reauthentication mechanism is able to
match the session with existing display and run in the proper context.

https://bugzilla.gnome.org/show_bug.cgi?id=782182