Restore direct display children to display layer

This undoes I78d4832b0ff7623684fc22fe1f464a02a1d91477.
Because now a display always has a real DisplayArea to provide
windowing layer (created by DisplayAreaPolicyBuilder).

Old:
 RootWrapper (DisplayContent#mSurfaceControl)
   > WindowingLayer (pure layer, not a WindowContainer)
     >> Child of display

 The surface of direct child under display uses WindowingLayer
 as parent.

Now:
 Display (DisplayContent#mSurfaceControl)
   > WindowingLayer (DisplayArea)
     >> Child of WindowingLayer

 The surface of direct child under display uses display as parent.
 Because the surface hierarchy is consistent with window hierarchy.

Bug: 326975721
Bug: 266243524
Test: AccessibilityOverlayTest#testA11yServiceShowsOverlayOnVirtualDisplay_shouldAppear
Change-Id: I7a1d09eb566e29e532d56627a9955692b55a5682