screenShield: Tweak curtain animation timings

Rationale:
 - Getting something out of the way should be quick;

 - Very few things in the real world move linearly so, linear
   animations, especially for something as big and visible as this,
   felt too artificial;

 - Moving the curtain out should start slower to make it feel like
   having weight (it fills the whole screen after all) but quickly
   accelerate towards the end to make it snappy too.

https://bugzilla.gnome.org/show_bug.cgi?id=686745