Add API to read and listen to duration scale changes for animations

Developers and accessibility services can remove animations by
changing the duration scale to zero. Physics-based animation
libraries like AndroidX's dynamic animations aren't duration-based,
but they need to know the duration scale to remove animations.

Expose the scale through ValueAnimator to avoid requests for
Context which may not be fulfilled by the developer.

Unhide test API getDurationScale and handle callbacks on the main
thread.

Test: atest ValueAnimatorTest
Bug: 221303983
Change-Id: I3134e0bf007df046a9a2aa0f9d866c27c7989e68