Background:
MMI code = any code you enter on your phone with * or #
USSD code = MMI code that doesn't only access the phone's hardware and requires a response from the operator

Here is how you currently send a MMI code from gaia:

      navigator.mozTelephony.dial(MMI_CODE, 0).then(obj => {
        return obj.result.then(function(result) {
          console.log('MMI response: ' + result);
        });
      }).catch(error => console.error(error));

It should be possible to do the same thing for a USSD code.
However, currently, the promise is resolved instantly with an empty result if the message is sent correctly, and the real response comes later, in a system message:

      navigator.mozSetMessageHandler('ussd-received', e => {
        console.log('USSD response: ' + e.message);
      });

Please note that USSD responses can also contain an object called USSDSession, that you can use to cancel or reply to a dialogue with the operator, which usually looks like a menu.

This PR tries to do the following:

• Fix a critical bug that appeared in 2.5, preventing any USSD session from working because they were cancelled everytime: https://bugzilla.mozilla.org/show_bug.cgi?id=1191205
• Silently cancel previous USSD sessions, so that there is no useless "Session expired" message : https://bugzilla.mozilla.org/show_bug.cgi?id=1198676
• Unify USSD and MMI promise behaviour, so that USSDSession methods and mozTelephony.dial() promises for USSD codes are solved once the response is received, along with a USSDSession object if needed. Unsollicited messages should still go in a system message.