[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). The decoding codeset
was changed from UCS-2BE to UTF-2BE as UTF-16 allows surrogates
(the use of two characters/four bytes to make one codepoint).

Signed-off-by: Petri Takalokastari <petri.takalokastari@oss.tieto.com>