Add the Dev Option for Desktop Experience

Add the Settings constants and the needed methods, including to update
the system property when the Settings changes.

Note: This should not be backed up.

Test: atest DesktopModeFlagsTest
Bug: 389092752
Flag: com.android.window.flags.show_desktop_experience_dev_option
Change-Id: I87af51b0df997236b166db6ed834d44a943dde96