[4/3] Introduce SpringTimings to avoid confusion.

These use floats as percentages of the animation, instead of explicit
durations in ms.

Fix: 372858592
Flag: com.android.systemui.shared.return_animation_framework_library
Test: atest TransitionAnimatorTest
Change-Id: I5caa7d43f808cb9beac0ba899a4fe5b84c1c70f6