manager: fade out explicitly locks more quickly

If the screensaver is explicitly asked to lock
(as opposed to locking because of idle), then
we should do it very quickly.  1 second is
too long.

This commit changes the fade to 60 steps per
second and a quarter of second long
(put another way, about 15 fade increments).

https://bugzilla.gnome.org/show_bug.cgi?id=668551