Add debug option to force desktop-first

This CL adds a system property to force a desktop-first mode on the
default display regardless of the proper decision factors.

Flag: com.android.window.flags.enable_display_windowing_mode_switching
Bug: 407869787
Test: adb shell setprop persist.wm.debug.force_desktop_first_on_default_display_for_testing 1
Change-Id: I9d1bfe3b6d6bf86d9a1653af0f93bf162908b582