Animate split divider

* TODO currently it doesn't animate when
dragging a split task back down because we need
to sync that with the other task scaling
animation playback controllers.
* It also doesn't animate in when opening a
split pair from overview because I think shell
is setting it to be visible before the divider
leash even makes it to launcher

Bug: 202914644

Change-Id: I6c60cffdb63aa262d0b547f85acc604c0c8a3637