[Dev option] Use Dev option to override whether DW should be enabled

Add utilities for dev option toggle in
DesktopModeStatus. Store an Int in Settings on manual override of
toggle.

Default value of this dev options solely depends on
enable_desktop_windowing_mode, which is intended

Toggle is shown only if DW is eligible on device

Currently this option only overrides features guarded by
enable_desktop_windowing_mode, later on it will be used to override a
few other DW features

Test: Added tests. Locally tested CUJs
Bug: 348193756
Flag:  com.android.window.flags.show_desktop_windowing_dev_option

Change-Id: I1c1f946b586a5ca0440b29879925ba1a624717aa