[PiP2] Fix NPE when create a split-screen

Fixed NPE thrown from PipTransitionHandler when DefaultMixedHandler
tries to query for PiP presense, root caused by the deprecated
PipTaskOrganizer reference when in PiP2.

Flag: com.android.wm.shell.enable_pip2
Bug: 375247956
Test: manual, create a split-screen when PiP2 is on
Test: manual, swipe a split-screen to home, with PiP-able Activity
Change-Id: I0ae1791f9ec6bbeaf59313b68653449ffa09d2fc