[Dev option] In DesktopModeFlags, cache a persistent value for

overrides stored in Settings.Global until reboot

This is so that it does not change with process restart.

Test: Added tests
Bug: 348193756
Flag: com.android.window.flags.show_desktop_windowing_dev_option
Change-Id: I71029e383392644516efc2c1520209217dce4858