Migrate DisplayContent.mOverlayContainers to DisplayArea

(1/n DisplayArea Sibling Roots)

Instead of adding magnification window to the overlay containers in
DisplayContent, move it to be part of the policy created display
hierarchy.

DA.Root
   - DA for FEATURE_WINDOWED_MAGNIFICATION
      - BELOW_TASKS
      - TDA
      - ABOVE_TASKS (below TYPE_ACCESSIBILITY_MAGNIFICATION_OVERLAY)
   - DA.Tokens for TYPE_ACCESSIBILITY_MAGNIFICATION_OVERLAY
      - Magnification overlay windows
   - ABOVE_TASKS (DA.Tokens's for layers above
     TYPE_ACCESSIBILITY_MAGNIFICATION_OVERLAY)

Fix: 156424590
Test: manual: magnification and screen rotation
Test: go/wm-smoke (manual)
Change-Id: I959c0a882b0a0e9a9dbb44ad62a3c65c1dfe3c06