Correct showing desktop experience dev option

1. Update isDesktopModeSupported to always ignore the availability on
   the internal display. If a device shouldn't be supported, let's not
   label it as supported at all.
2. Add the necessary method to DesktopModeHelper
3. Update WindowManagerService to force-disable the old developer option
   when the new one is shown
4. For consistency, don't test the developer option in DesktopModeFlags
   if DesktopExperienceFlags is disabled (e.g. when content mode
   management is enabled).

Bug: 389092752
Test: manual
Flag: com.android.window.flags.show_desktop_experience_dev_option
Change-Id: Ib8a6c8bf236a1c04e60ed26ea7ecb5a93176de61