[tones] Remove dead code related to tones w/o user connection

oFono and our modem simply don't give such detailed information on the audio path.
[call] Play local alert tone if network certainly does not do it

Use oFono VoiceCallAgent API to get notified when a local alert tone (ringback tone) needs to be played.
Merge pull request #19 from marttipiirainen/ringback_tone

[telepathy-ring] Play local ringback tone according to oFono signalling