Keep split on top when going to Pip if not swipe to home

* If main or side stage have a child, we move that stage to
be on top when performing the split to pip transition.
* We also prevent the stage with a child from dismissing in
DefaultMixedHandler#animateEnterPipFromSplit
* In both cases we use isSplitScreenVisible() as our proxy to determine
if we're dismissing one of the split pairs vs swiping up to home,
since in the swipe up case split becomes invisible and then Pip requests
to be shown as a separate transition

Bug: 293254265
Test: Steps don't repro. Pip shows up and hides when it's supposed to.
There's a KI quickswitch issue but happens w/o this change
(b/303722342)

Change-Id: I7892ac1612ab2dccdd81c0bece987aab3ad69194