Desks: Do not deactivate on minimization of last task

When the last task of a desk is deactivated, WM core reports both the
task and its former root task (desk) as TO_BACK changes in the
transition. This doesn't mean the root is actually behind anything
unless something else was moved to front, so don't deactivate the desk.

Flag: com.android.window.flags.skip_deactivation_of_desk_with_nothing_in_front
Fix: 421124396
Test: open 1 task in desktop, minimize it - verify desk remains active
and taskbar pinned

Change-Id: I4a3426264932ab3734b9eeeeb0a0ede5fe8a0efc