rilmodem: Support for UCS-2 in USSD

Support to send and receive USSD strings that cannot be codified using
the GSM alphabet.
src: Support to send UCS-2 codified USSD strings

Core ofono always codified USSD strings using plain GSM alphabet.
This change uses UCS-2 in case GSM alphabet is not enough.
Merge pull request #83 from alfonsosanchezbeato/ussd-ucs2

USSD - UCS-2