MNCC: Represent 'keypad' as one-character charstring, not int

The MNCC Unix Domain Socket encoding uses an int here, which is
a bit odd, given that it's an ASCII / IA5 char value on the actual
GSM L3.  Let's convert from/to something useful.

Change-Id: Id17ac502ca33f4962214a3d5938d0dc29ca6ec1b
2 files changed