[rilmodem] Add support for ReleaseAndAnswer()

Add support for ReleaseAndAnswer() which is needed for BT headset that
sends AT+CHLD=1 to bluetoothd. Currently telepathy-ring is not using this
feature.
Tested with release-and-answer test script.

Signed-off-by: Petri Takalokastari <petri.takalokastari@oss.tieto.com>
Merge pull request #86 from ptakalok/master

[rilmodem] Add support for ReleaseAndAnswer()