[Dev options] Use Android.os.SystemProperties instead of
java.lang.System.setProperty for caching toggle overrides.

java.lang.System properties are not shared amongst processes

Also refactor DesktopModeFlags and DesktopModeFlagsUtil.

Test: Updated tests
Bug: 348193756
Flag: com.android.window.flags.show_desktop_windowing_dev_option
Change-Id: Icf019318e11f0a1c3767fb8c620328d24384a193
5 files changed