Disable the dev option if the feature is already enabled.

Instead of allowing the user to enable it further, just disable and mark
it as already enabled.

Fix: 407747957
Flag: com.android.window.flags.show_desktop_experience_dev_option
Test: atest DesktopExperiencePreferenceControllerTest
Change-Id: Ic75b39537a3c5ffa851bf2e7cfe3e5c0d46a3a7b