[PiP on Desktop] Reparent expanding PiP to root desk for multi-desks.

Also update windowing mode to UNDEFINED if PiP is expanding to Desktop
task and multi-desks flag is enabled.

Bug: 416355546
Bug: 403345629
Test: atest PipDesktopStateTest DesktopPipTransitionControllerTest
Test: Manual - expand PiP while in Desktop, bring another task to front
and verify that when tapping on previously PiP-ed task on app handle, it
is brought to front
Flag: com.android.window.flags.enable_desktop_windowing_pip

Change-Id: I4425e7239c87a386a21b237358f7fecd71c25a8a