Hide leaf-tasks at end of swipe-to-home animation

This is needed for split-screen. Since their leaf tasks
are not "independent", Transitions.java will not hide them
in the finish-transaction (it shouldn't because it doesn't
know if the parent is doing the hiding). In recents' case,
though, we know that the tasks will be hidden at leaf-level
(due to being reparented out of split).

Without hiding here, there will be a flicker after it gets
reparented out of the animation leash, but before the app
visibility is committed in WMCore.

Bug: 228883780
Test: Enter split, swipe-to-home.
Change-Id: I9d0fa1542cb82aabbc89998368864107cee56c72