Bluetooth: host: prevent registering services in the wrong context

There is a touchy spot where registering a service might result in sending
service changed indications that are not necessary. This is due to a race
condition between the SC work and the GATT DB hash being calculated.

Only allow users to call this API when we know it is safe:
- either before BT is initialized
- or after settings have been loaded (and hash calculated)

Fixes #54047

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>