Fix issues with MMI/USSD codes.

There a few issues here:
1. The queuing of MMI codes for handUssdRequest was not working properly.
I have removed it in favor of returning an error if there is an ongoing
MMI code.  The functionality was pretty broken and despite my efforts I
could not sort all of it out well enough to be confident that it could be
fixed.
2. There was no attempt to retry handleUssdRequest calls on IMS.
3. There was a missing fallback to CS for regular MMI requests.

Test: Manual
Bug: 37484804
Change-Id: I8b2ed5ed9c29cdf95532096d60487656946b4cc0