Use RectFSpringAnim for auto-enter-pip

Make SwipePipToHomeAnimator a subclass of RectFSpringAnim and use the
bounds calculated from latter for auto-enter-pip transition.

Note that the landscape to portrait case may not fully comply to
RectFSpringAnim as we calculate the position separately in
SwipePipToHomeAnimator.

Video: http://recall/-/aaaaaabFQoRHlzixHdtY/CWBPU2A6vB8CeTFRanQLx
Bug: 184789411
Test: manual, see video
Change-Id: I67dae09e682b7d8f1a1f08995eb3bcf76bdf7417