Move windowing mode switching to DesktopDisplayModeCtrl

This CL is a pure refactor to move out the display windowing mode
switching-related logic to DesktopDisplayModeCtrl.
More related logic is incoming so separating the class prevents the
original handler from getting too complicated.

Flag: com.android.window.flags.enable_display_windowing_mode_switching
Bug: 397557203
Test: DesktopDisplayModeControllerTest

Change-Id: Ib4323517e31ca8ebb79642b84a6b937e85c12363