Allow disabling Dark mode in the app

This issue only needs to be tested in 1 device. Login as admin in a site with  MDL-67806 applied. Go to Site administration > Mobile app > Mobile features > Disabled features option. Select the "Dark mode" option and save the changes. In the app, before entering any site go to Settings > General and select "Dark mode" in Color Scheme. Login in the site from step 1. Check that the theme changes to "light" when entering the site. Go to App Settings > General and check that the "Color scheme" is disabled and there is a message saying the setting is forced. Logout. Check that the theme changes to dark again. Enter the site again. Check that the theme changes to light again. In web as admin, unselect Dark mode. Restart the app or do a logout/login. Check that the theme changes to dark.