kded: Add startup settings manager

This adds a new component that manages Plasma Mobile specific configuration on every session startup. It also restores configuration when logged into a desktop session.

This allows us to remove https://invent.kde.org/plasma-mobile/plasma-phone-settings, as well as configuration that was set in the look and feel. This also gives us an easy way to control configuration upgrade paths, and in the future, add ways for the configuration to easily be reset for debugging purposes.