Always use a DisplayArea for WindowingLayer

This fixes the case if FEATURE_WINDOWED_MAGNIFICATION is supported:
 Correct hierarchy
  Display 0
   > WindowedMagnification:0:31 (WindowingLayer)
   > HideDisplayCutout:32:35
   > Leaf:36:36
   > Overlays

 Wrong hierarchy after calling migrateToNewSurfaceControl
 (The DisplayArea 32~36 should not belong to 0~31)
  Display 0
   > WindowedMagnification:0:31 (WindowingLayer)
     >> HideDisplayCutout:32:35
     >> Leaf:36:36
   > Overlays

There are various assumptions that the real parent surface should be
used as the target of reparent except display. Such as onParentChanged
and animation leash management. So it might not be appropriate to be a
general override in WindowContainer.

To reduce confusing of WindowingLayer, this change makes the layer
name consistent with its purpose and window hierarchy. There will
be a real DisplayArea for WindowingLayer, so the exception case of
relationship between surface and container is gone.

Before:
 RootWrapper (DisplayContent#mSurfaceControl)
  > Display 1 (WindowingLayer)
    >> TaskDisplayArea
  > Overlays

After:
 Display 1 (DisplayContent#mSurfaceControl)
  > WindowingLayer
    >> TaskDisplayArea
  > Overlays

Bug: 326975721
Test: atest DisplayContentTests#testValidWindowingLayer
            DisplayAreaPolicyBuilderTest
Test: Kill systemui (which triggers setOrganizer ->
      sendDisplayAreaVanished -> migrateToNewSurfaceControl)
      and check surface hierarchy.

Change-Id: Id4c2723a67738827be81ffb6fd4f47cf3a094634