[nemo-system-settings] Implement synchronous initialisation of LocationSettings. Contributes to JB#38371

The gps flight mode / power state functionality of LocationSettings is
provided by connman (libconnman-qt) via an asynchronous interface.
This commit allows the client to specify a flag which will force the
initialisation to occur synchronously.

Also encapsulate private data in private class to allow changes to be
made without affecting binary compatibility etc.

Contributes to JB#38371