[CS] Add WindowRootViewComponent to get access to the WindowRootView.

This pulls out the WindowRootView from CentralSurfacesComponent so that
we can finish removing CentralSurfacesComponent. The doc on
WindowRootViewComponent explains its existence and why we can't just
inject WindowRootView into NotificationShadeWindowController directly.

Bug: 277762009
Test: smoke test of the shade
Test: atest NotificationShadeWindowControllerImplTest
Change-Id: I8e67af46383a075688ac68c66fa641439e2079c5