Have DisplayContent be the call point for assigning window layers

First step in trying to make the WindowList private to DisplayContent
Have the rest of the system call DisplayContent when they need to
assign layers to windows instead of calling WindowLayersController
directly. That way all the various call points don't need to get
the WindowList from DisplayContent to pass on to WindowLayersController.

Test: Existing tests pass.
Change-Id: If869abf7ba1c33b575908006ae5ad1557abc361c