Polish home-key from split to pip transition

Make pip transition handler respect abs start bounds in the transition
info to determine a proper start bounds for entering animaiton. All the
leaf tasks will be default alpha = 1 after applying finishTransaction,
so makes sure to hide the dismissing tasks as it might be reparented in
startAnimation due to auto-pip. Also hide the divider bar with startT
since it no longer needed for playing dismissing animation.

Bug: 281476331
Test: repro steps of the bug
Test: pass existing tests
Video: http://recall/-/aaaaaabFQoRHlzixHdtY/hRGDB2rPkc6GQSpugcScTr
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:193b3de8c2f7ac36b63cc825e6ae02d68f0e3e1e)
Merged-In: I361a607e2dffa7c2688ccc0720c04b7facea4830
Change-Id: I361a607e2dffa7c2688ccc0720c04b7facea4830