Add support for SystemUI Window Management

Enables SystemUI to create/position its own ui elements.

First of all, this adds, to WM, the concept of a ShellRoot
which represents a piece of the hierarchy that a client shell can
do whatever it needs with. For now, multiple of these roots can
be registered at various "levels" (which correspond to window
types for now). This is needed because not everything will live
in this piece of the hierarchy, so handling z-order will still
be a shared effort between the Shell and WM for a while.

On the SystemUI side, a new Dependency called SystemWindows
provides simplistic window management for these system-ui
windows via WindowlessWindowManagers per-display-per-layer.
The benefit of this is that manipulation of these windows lives
entirely in SystemUI making synchronization easier and making
it possible to move a lot of the special handling code out
of wm (eg. DOCK_DIVIDER). As a result, SystemUI becomes
more customizable and WM becomes simpler.

Early clients of this are going to be display-level IME
handling and Split-screen.

Bug: 133381284
Test: manual test after later CLs
Change-Id: I1602d9b9b69d38b9ff15806e509cc8128c837748