Use initial velocity with spring animations in TransitionAnimator.

I've also reworked the test a bunch to reduce repetition and use
parameters better. Shouldn't need to touch it much after this.

Bug: 323863002
Flag: com.android.systemui.shared.return_animation_framework_library
Test: atest TransitionAnimatorTest
Change-Id: I961198763a73a58b3b3db0fa8b3f79f53cbf1c98