[voicecall] Inform client when local ringback tone is needed

In some networks, MO calls do not get the "alerting" tone in-band, so the device needs to play one locally.

This commit adds a "voice call agent" API. It currently contains only the ringback tone notification, but can be extended later.
[provision] Expose provision_get_settings() in header (for testability)
[unit] Add provisioning tests
[provision] Fixed memory leak
[mbpi] Parse gsm provider name
[provision] Take SPN into account when finding APNs
Merge pull request #214 from marttipiirainen/ringback

[voicecall] Inform client when local ringback tone is needed
Merge pull request #213 from marttipiirainen/mbpi3

Take Service Provider Name into account in MBPI provisioning