If this isn't initialized and an idle watch gets instanced before
meta_idle_monitor_native_reset_idletime() gets called, that idle watch
would get triggered as soon as we hit the main loop.

This was causing gnome-session to go into idle mode at session start
thus making gnome-shell lock the screen.

In the past this bug was being masked by either logind emiting
session active signals or a stray input event making it through at
startup.

https://bugzilla.gnome.org/show_bug.cgi?id=772839