Integration of back gesture animation on Sysui (BackNav 2/n)

This CL contains the BackAnimationController, on WMShell, which
controls the gesture based animation.

When the gesture is done, the animation is done. It will be handed over
to a TransisionHandler (in the next CL)

Test: atest WMShellUnitTests:BackAnimationControllerTest
Bug: 131727607
Change-Id: Ifc2ed99bfd51079f12a83468ca44e4aa19460e47