Fix Unicode string handling

Linux uses UTF8 but java and MTP both
use UTF16. In a few places, this results
in the top byte of a UTF16 string being
truncated on conversion to UTF8.

Also, the hardcoded UTF to byte serialization
in MtpStringBuffer is incorrect.
Replace it with conversions from std, and
replace usages of MtpString with MtpStringBuffer.

Remove any remaining usages of libutils
and replace them with corresponding std
libraries.

Bug: 70546563
Test: Mtp works, tests pass
Test: file/folder names containing emoji can be transferred to/from
windows
Change-Id: Idbcb73f1beac17ce8a90843fa254e759dd1a6369