Animation constructor easing

Fix bug https://github.com/web-animations/web-animations-next/issues/131.

Timing functions weren't working with explicitly constructed Animations because normalizeTimingInput was being called twice.

Now we just store the raw timing input in the Animation object, and call normalizeTimingInput any time we need a normalized timing object.

@shans ptal
We don't want to be normalizing here if possible as this function gets called multiple times per frame.