[ofono] Encode SMS as UTF-16 instead of UCS-2

UCS-2 is an older 16-bit encoding compatible with the unicode BMP.
UTF-16 extends UCS-2 to add support for surrogate pairs and the rest of
the unicode set. All valid UCS-2 text is also valid UTF-16 text, and all
UTF-16 text not containing surrogate pairs is valid UCS-2.

We decode incoming SMS as UTF-16 instead of UCS-2 to add support for
these extended characters. We should do the same for encoding outgoing
SMS messages.