fadeInSurfaceBehind() when the remote animation starts.

This should never have been done earlier, it worked
thanks to null-checks in the animator. This change
consolidates all of the surface-behind animation
logic into notifyStartSurfaceBehindRemoteAnimation
where it logically belongs.

Bug: 229890190
Test: atest SystemUITests
Change-Id: Iee97d46929d6de1ad3f9e0373be584d48ced440c