WindowManager: Add basics of generic overlay support.

WindowContainer is extended with basic support for overlay at any
level of the hierarchy. Overlays are provided via a
SurfaceControlViewHost. Using SurfaceControlViewHost
instead of WindowState lets us flexibly attach at any level of the
hierarchy. We forward configuration changes over the new
SurfaceControlViewHost side-channel. WMS grows add and removeTaskOverlay
as specific instantiations, to be called first by game overlay
service.

Bug: 213603716
Test: Existing tests pass
Change-Id: I82c427ad9cd42cbbc0c994afdfcf32be39370164