Unicode: specify destination length in utf8_to_utf16 methods

String16(const char *utf8) now returns the empty string in case
a string ends halfway throw a utf8 character.

Bug: 29267949
Clean cherry-pick from 1dcc0c82394ec9cd6887c7ca39f9b5024db01ac9

Change-Id: I5223caa7d42f4582a982609a898a02043265c6d3
4 files changed