Stop using DesktopModeStatus in SystemUI

Create a new `desktop` module, currently only to inject desktop-related
classes into SystemUI.

Also move the fakes for DesktopState and DesktopConfig to the main
folder, so they are accessible from SystemUI.

Bug: 395863348
Test: atest LauncherProxyServiceTest
Test: Build and run
Flag: EXEMPT (refactor)
Change-Id: Id013a6f084846766a2e59684009db46e3685ae1e