telephony: Fix issue in short code ussd detection

According to the 3PGG TS 22.030 specification
Figure 3.5.3.2: A 1 or 2 digit "short code" is
treated as USSD if it is entered while on a call or
does not satisfy the condition (exactly 2 digits
&& starts with '1').

Following rule is already addressed in function
GsmMmiCode::newFromDialString.

If the user of the device enters one digit followed
by the #-key, phone shall initiate a USSD/SS command

Change-Id: I70795da1fb5144d1c91059f6200b74b5fd33de22
Author: Jeevaka Badrappan <jeevaka.badrappan@intel.com>
Signed-off-by: Jack Ren <jack.ren@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>
Author-tracking-BZ: 28800