1) Factors InkWell out of Material so that components can use an ink well
   without needing the shadow/level machinery.

2) Makes the ink effect move at a different velocity once the tap has actually
   occurred, converging with the spec. We don't have the right speeds yet, but
   at least we're approaching the right shape.

3) To support (2), added a primaryPointer attribute to GestureEvents to let
   authors coorelate gesturetapdown events with later gesturetap events.

4) To support (2), modernized SplashAnimation to used AnimatedValue and friends.

5) Added more constants to view-configuration.dart that match Android.

I've also removed the cancelling of the ink effect on scroll. The proper way to
do that is to notice that someone in the event chain is listening for
scrollstart and delay the beginning of the ink effect for some period of time.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1019023003