icon: update icon/asset for settings
Right now the shell is using preferences-system-symbolic icon for the button that takes you to the system settings. That icon, based on the icon naming spec, was meant for something we don't use anymore, aluncher categories. It just happens to have matched the metaphor of the Settings. The app icon is actually org.gnome.Settings-symbolic.
The metaphor of the Settings app icon has changed for 3.30. We can either depend on the app icon or ship a local asset that matches Settings.