Intruduce Cross Activity Animator

Add a new animator to support `TYPE_CROSS_ACTIVITY` when starting a back
navigation and it will return to another activity.

This also introduces the background controller that could show the
animation background surface attached to the display area.

Bug: 236759828
Bug: 238474994
Test: atest BackAnimationControllerTest BackNavigationControllerTests
Change-Id: I6574b6ce969cb905ba8b4a797560216aed2ee3cb