[smsutil] Improve conversion of incoming UCS2 SMS to UTF-8

By this change characters found from incoming UCS2 SMS which are not part
of UTF-8 character set are passed to message manager API client as
Unicode escape characters (\uxxxx or \Uxxxxyyyy).

Signed-off-by: Petri Takalokastari <petri.takalokastari@oss.tieto.com>