Fix NPE in updateDisplayWindowingMode

This CL fixes the crash issue in CTS which uses non-trusted overlay
displays.
Such a display is not “organized” by the DisplayOrganizer so
DisplayAreaInfo can be null.

Bug: 406106717
Flag: com.android.window.flags.enable_display_windowing_mode_switching
Flag: com.android.server.display.feature.flags.enable_display_content_mode_management
Test: android.server.wm.multidisplay.MultiDisplaySystemDecorationTests
Change-Id: I2dd81510a73736eeb0392256349f9619e5a764a1