Bug 861725 - B2G Emulator: support call forwarding

Please see [bug 861725](https://bugzilla.mozilla.org/show_bug.cgi?id=861725).

This PR implements:
1. RIL_REQUEST_QUERY_CALL_FORWARD_STATUS [1]
2. RIL_REQUEST_SET_CALL_FORWARD [2]
3. AT+CCFC [3]

[1] https://github.com/mozilla-b2g/platform_hardware_ril/blob/master/include/telephony/ril.h#L1683-L1704
[2] https://github.com/mozilla-b2g/platform_hardware_ril/blob/master/include/telephony/ril.h#L1707-L1720
[3] TS 27.007 session 7.11