[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.
Merge pull request #214 from marttipiirainen/ringback

[voicecall] Inform client when local ringback tone is needed