Re-implement Z-ordering based off SurfaceControl hierarchy.

First steps in using the SurfaceControl hierarchy pervasively through
the WindowManager. For each level of the WindowContainer hierarchy
we instantiate a buffer-less SurfaceControl which is used to apply
constraints to all of it's children. For now we use full-screen surfaces
at every non-leaf level and so the only constraint we apply is Z-ordering.

Calculation of Z-ordering is done through the WindowContainer#assignLayers
pass and begins with DisplayContent. Changes are accumulated in-to a pending
transaction which is applied as part of prepareSurfaces (which now applies
to each level of the WindowContainer hierarchy rather than just windows).

Bug: 64815723
Test: Existing tests pass. go/wm-smoke. ZOrderTests.
Change-Id: Ib31bc3107c7fa398cf2ed72430fcb7596fad6fd5