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
This fixes an issue analogous to bug 760330 for the X11 backend,
except on this backend we wouldn't crash accessing free'd memory.
Instead we're leaking watches since we steal them from the hash table
which means that when they're removed in
_meta_idle_monitor_watch_fire() they're no longer there and thus
they're never free'd.
https://bugzilla.gnome.org/show_bug.cgi?id=760476
