New TimeInterpolator interface for android.animation package.

The new animation package's reliance on the old Interpolator interface (in
android.view.animation) was an eyesore. Adding TimeInterpolator, and having the
old Interpolator interface extend it, allows the new Animator classes to break
the tie to the older animation package completely. However, developers can still
use the older Interpolator-based classes, such as AccelerateInterpolator,
because they all implicitly extend the new TimeInterpolator class.

Change-Id: I41132fa56167ba564f4839113289114d0ea31a92