Make sure tasks not in split root is invisible while in split

When in split mode, home task will be reparented to the secondary split
and leaving tasks not supporting split below. Because TaskDisplayArea
always adjusts home surface layer to the bottom, this makes sure tasks
not in split root won't occlude home task unexpectedly.

Bug: 187153990
Fix: 185872271
Fix: 159767867
Test: atest RootTaskTests
Test: atest com.android.wm.shell.flicker.legacysplitscreen
Test: dock translucent task to split primary while having task not
supporting split in recent, recent app won't be occluded.
Test: primary task won't be invisible while in split minimized mode.

Change-Id: Iaff7e1810d87e5b724129c615c112cfa22dc5c36