Handle drag-to-deskop of translucent tasks

If a translucent task is dragged to desktop, a visible task behind it
may be included in the transition changes. Instead of throwing an
exception because that state was unexpected, this change layers it at
the bottom during the animation and caches it to be restored in the
drag-cancelled case.

Bug: 317116172
Test: open Chrome fullscreen, open Nearby Share dialog from quick
settings, start drag-to-desktop gesture - verify no crash on both the
commit and cancel cases

Change-Id: Ica05c8c6798c66d3d1e612fc97efcce4bdf63b80