Spoof response to AT+CNUM if we don't get a reply from the audio gateway.

The Huawei Honor 4X CHE1-CL10 phone doesn't reply to AT+CNUM so
we end up disconnecting the service level connection. In this CL,
I'm spoofing an OK response to AT+CNUM in case we time out waiting
for a reply.

Bug: 26504036
Change-Id: I91d5ab4ea91fd67b3f6c6f796c39fd0091bff5cf