Adds followAnimatedTargetAlongPath, and modifies the expanded controller to use it.

This also moves the end action methods into the controller, which is a more logical place for it. That way, PhysicsAnimationLayout only has one public method (setActiveController). This sets us up better for future abstractions that don't rely on a custom view at all.

(also updates the docs for these changes and several recent ones)

Test: atest SystemUITests
Bug: 134077101
Change-Id: I6fcc14587b07f14371fa75fbbe0cc31353aa5c0b