objectManager: handle proxies coming and going

objectManager: handle proxies coming and going

Ever since commit b8e29ae8c78658917c8a8ddd391667d7d9e36971
(I think), start up is littered with this message:

 Gjs-WARNING **: JS ERROR: could not get remote objects for service
 org.gnome.SettingsDaemon.Smartcard path

since gnome-shell is now started before gnome-settings-daemon.

This commit addresses the problem by making the object manager code
not try to autostart its proxy, and instead wait for it to appear.

https://bugzilla.gnome.org/show_bug.cgi?id=772589