Move exit PiP leash to the transition root

Before, when exiting PiP to previous Task, the PiP leash can be the
activity leash cropped by its parent leash. Now, we reparent the PiP
leash to the transition root to make sure it is not covered.

Bug: 207070762
Test: manually verify with ActivityEmbedding PiP app
Change-Id: Iab8671d87c4369c6a090b06cfa71736ae9ba5a2b