Rephrase BackAnimationController and BackAnimationRunner

This CL did some refactors include:
- Make the back animation will follow these two phases to better
  understand.
  1. Phase 1 animation will start from gesture start, and end when
     gesture released.
  2. Phase 2 animation aka post commit animation will start after gesture
     released.
  3. When the animation is finished, make sure we could trigger the real
     back behavior and notify the core.

Bug: 238475694
Test: atest BackAnimationControllerTest
Change-Id: I251b9674a4aa9965c338ccf592efaeabdcecd5e8