Restore direct display children to windowing layer

On virtual displays, there is no magnification DA, so TDA
is a direct child of DisplayContent. However, DC *still* makes
a root-wrapper surface in this case, so we have to restore to
DC.getWindowingLayer() instead of DC.getSurfaceControl() since
mSurfaceControl gets replaced after construction.

Added a comment in makeChildSurface to explain that the
mSurfaceControl used there is not the same after construction so
that others will hopefully be less confused by this.

Bug: 266243524
Test: AccessibilityOverlayTest#testA11yServiceShowsOverlayOnVirtualDisplay_shouldAppear
Change-Id: I78d4832b0ff7623684fc22fe1f464a02a1d91477