Implement the new PiP animation (fade-in).

The new fully gestural navigation changes how user goes home from an
app as well as the animation of the app transition. The current PiP animation (bounds) is not compatible with the new model because of the direction of movements. Instead of animating bounds, we can fade the PiP window in after app closing animation finishes.

Fixes: 122609330
Test: 1. Open youtube, play a video, tap home to observe the new PiP
animation (demo video attached in the ticket b/122609330) 2. With gestural nav on, observe pip transition when swiping up to home.
atest ActivityManagerPinnedStackTests
atest BoundsAnimationControllerTests
atest RecentsAnimationTestTest

Change-Id: I28eeb1aa99c4fd569845ca7a42561f6b20796f9b