DTMF.StopTone does not work correctly

Submitted by Kai Vehmanen

Assigned to Pekka Pessi @ppessi

oFono does not currently (as of 0.38/git-master) support any ways to stop sending DTMF tones.

Thus, unless oFono APIs are changed, ring's Channel.Interface.DTMF.StopTone() should return NotAvailable as specified in: http://telepathy.freedesktop.org/spec/Channel_Interface_DTMF.html#Method:StartTone

Alternatively ring could implement MultipleTones() as that more closely maps to oFono APIs.

Version: git master