How to reverse transition animation, rather than canceling it, on back press during transition?

Hi, I found this annoying behaviour, where if you press back during transition, the push animation gets canceled, rather then reversed.

For Activities behaviour is that if you press back during animation it will be reversed from the current position in the animation.

In conductor this is not possible because if you press back during transition, `AnimatorChangeHandler.getAnimator` `from: View?` is null, hence you cannot create a animator from current values backwards.

Start at AnimatorChangeHandler.getAnimator and reproduce pressing back during an in-progress push transition. Compare the current Conductor behavior with the described Activity behavior and trace how the transition state is handled when `from` is null. Done means the transition reverses from its current position instead of being canceled.

Written by the indexing model from the issue text.