USSD: support direct responses

Initial issue: #7 (closed).

The known issue in !33 (merged) is that direct responses to the received USSD messages are not working yet. Right now it's already possible to send responses manually initiating new USSD requests adding the code after the * mark. For example:

*123# shows a message: 1 -- option A; 2 -- option B.

It is possible to select option B with a new request *123*2# manually. Fixing the issue means we should be able to reply directly with a 2 code from the menu.

While the UI and the ofono commands handlers are already introduced, it needs additional debugging. Send response ofono command is not working for me since I'm receiving status change from user-response to idle from the modem immediately after the initial USSD request finished. So the Respond command is not working as expected.