SurfaceControlViewHost: Basic child window support

WindowlessWindowManager already has some support for multiple
windows so the main thing we need to do is just ensure that the
Windowless session is injected in to the ViewRoot of things attached
to our token. We can do this via some somewhat unsightly glue
code in WindowManagerGlobal.

Bug: 218615203
Test: SurfaceControlViewHostTests
Change-Id: Icce97c02b6155a7ef368821670db26728b95b6b9