Refactor and generalize SpringAnimationHandler.

SpringAnimationHandler is only responsible for motion tracking and
updating the list of springs it is handling.

Spring initialization and updates are handled by the AnimationFactory
interface.

This is done in preparation for future CLs where SpringAnimationHandler
will be used in more cases.

Bug: 38349031
Change-Id: I5ebce710cc05e4e88b708ceea5d2cfefbd0eb3d0