screenShield: Compute lock timeout fade duration using animation settings

screenShield: Compute lock timeout fade duration using animation settings

When the screen is marked as idle, we normally start a fading animation and
a timeout to finally lock the screen. This timeout is configured using the
fade time if no longer delay is set in settings.

However if animations are disabled or slowed-down/up, the fade time is
different from the STANDARD_FADE_TIME and so we might end up showing the
lock shield without actually locking for STANDARD_FADE_TIME in the disabled
or slowed-up animations case, or locking too early in case of slowed-down
animations.

So, just adjust the timeout time using the same logic of animations so that
this value is matching all the times.

Related to https://gitlab.gnome.org/GNOME/gnome-shell/issues/1744

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/749