Fixes #1170

AnimationPerformance had some logic for supporting multiple variables
that was hardly ever used. ValueAnimation, a subclass, has logic for
handling a single variable. I've removed the logic for handling
variables from AnimationPerformance in favour of most call sites instead
using ValueAnimation.

Move the animation libraries into src/animation and change importers to use
package:sky/animation.dart. Also, move scheduler.dart into the animation
library so that the animation library can be self-contained.

We still need to tune the various parameters, but this patch is a start.