Return the actual interpolator of the ViewPropertyAnimator

The implementation of getInterpolator() was always returning null
(probably a quick copy-paste from the default Animator implementation).
This patch fixes the problem by returning the interpolator set by
setInterpolator(TimeInterpolator) or the default one if none has been
set yet.

This patch also avoid creating multiple instances of ValueAnimator in 
order to retrieve some default values.

Change-Id: I8880f419f021a8b980fb32bebe927915fde19bf7