Rework application visible state handling

Signed-off-by: Simon Busch <morphis@gravedo.de>
Load WiFiManager extension only for settings app

Due to it's nature the current WiFiManager extension can be loaded only once per process
so we load it only for the settings application until we introduced a separate service
component for it again.

Signed-off-by: Simon Busch <morphis@gravedo.de>