Allow sending of in-call MMI codes through to telephony.

1. Fixing bug where having multiple calls in progress it would not be
possible to send the ECT MMI code due to code that checks for the number
of current calls.
2. Fixed issue where InCall MMI codes would be dialed as a regular call,
showing the InCall UI.  In KitKat, dialing an incall MMI code would
bypass the InCall UI, dialing the MMI code directly.

Bug: 17700763
Change-Id: I41b9b329609e100540c4d60e1243dc08354bb481