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 store the raw timing input in TimedItem.timing, and the normalized timing in TimedItem._timing.

Timing is normalized at creation time for explicitly constructed timed items, and at play time (in scope.AnimationTimeline.prototype._play) for timed items created using Element.animate.

@shans ptal