Check last parent when calculating Shell transition root

Before, we pick root leash only based on the end state hierarchy, which
may result the reparent animation covered by other windows between the
new parent and old parent. Now, we make sure the root leash is also the
ancestor of the old parent.

Bug: 207070762
Test: atest WmTests:TransitionTests#testCollectReparentChange
Test: manually verify with ActivityEmbedding enter PiP from split screen
Change-Id: Ieeb2ce19b47adf4223ba6d58a3be10d7079f4eb3