[PiP on Desktop] Reorder parent to back for multi-activity PiP.

Recall: http://recall/clips/ee10466e-2115-4deb-8d1e-2a2d658435ef

For multi-activity PiP in Desktop, the parent activity is brought to
front and shown when the child activity breaks off into PiP (if we are
remaining in Desktop session).
This CL explicitly reorders the parent to the back to prevent it being
shown during PiP. Also adds a collect call for the parent before
bringing it to front when expanding from PiP, so that it is not shown
before the animation.

Bug: 409480429
Test: atest DesktopTasksControllerTest
Test: Manual - launch Netflix in Desktop with another task open, then
enter PiP. Verify that Home activity does not reappear
Test: Manual - verify that direct-enter PiP from fullscreen for
single-activity and multi-activity are WAI
Test: Manual - verify other multi-activity PiP expand cases e.g.
pip-to-split are WAI
Flag: com.android.window.flags.enable_desktop_windowing_pip

Change-Id: I1be190619dfa909591999cb61d0cbdfbe4ea72b6