Move home task under the fullscreen task...

when moving to fullscreen so that using back navigation results in user
ending up in home rather than desktop. It doesn't reorder home behind
the task for freeform by default displays.

Bug: 392621916
Test: Manual & atest DesktopTasksController
Flag: EXEMPT Bugfix
Change-Id: I52be442e8dd050950e1f89e9c3e4580cd0150366