Bug 861725 - B2G Emulator: support call forwarding

Please see [bug 861725](https://bugzilla.mozilla.org/show_bug.cgi?id=861725).

This PR implements AT+CCFC command [1].

[1] TS 27.007 session 7.11
```
if (ret) {
  return "+CME ERROR: 3";
}

return "OK";
```