Update display windowing mode when default display is added

This CL ensures that the display windowing mode of the default display
is updated when it’s added (i.e., right after SysUI’s initialization).
Otherwise, it’s never updated if any external display or input device is
connected to the device. Note that this is usually fine because the
display windowing mode remains the same throughout the user journey
without external displays or input devices, but in some edge cases
(e.g., SysUI crash followed by display disconnection, or
FORCE_DESKTOP_FIRST_ON_DEFAULT_DISPLAY debug flag) it can be an issue.

Flag: com.android.window.flags.enable_display_windowing_mode_switching
Fix: 431975631
Fix: 432592973
Fix: 438605732
Test: DesktopDisplayEventHandlerTest
Change-Id: Ie99e2d3594e5028cb12470071d0d4da006e78f9f