Unify USSD and MMI promise behaviour

[bchabod]

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.

[hsinyi]

Hi Ben,
Thank you very much for the WIP!

Here's some context about the current USSD API design. We need to rely on an additional way to notify gaia "ussd responses" because the responses are coming from distinct unsolicited events per android ril.h[1]. Hence, it lacks a determined link b/w the response and a request. So App needs to rely on "ussd-received" notification. Previous discussion here [2].

[1] http://androidxref.com/5.1.1_r6/xref/hardware/ril/include/telephony/ril.h#2195
[2] https://bugzilla.mozilla.org/show_bug.cgi?id=897773#c7

[moz-hwine]

Please do not file PRs here, see https://developer.mozilla.org/docs/Mozilla/Developer_guide/How_to_Submit_a_Patch