Launch split tasks at beginning of animation instead of the end

* Ideally we animate the live leashes from the placeholder tasks,
that's a lot more changes at this moment so we don't do that
* Instead we just hold off fading in the leashes until the very
end of the animation
* This way taskbar can scale down correctly in both contextual and
app pair launch cases since now launcher visibility will be changed
at the same time

Bug: 308851855
Test: Launches are a bit snappier from workspace/all apps compared
to overview, but in a good way!(?)

Change-Id: I73a9284a2b1a993c5a020396e2aca8499fc5eb09