system/core: using equivalent code to fix build break for win host

Error from x86_64-w64-mingw32-g++ was:
system/core/libutils/Unicode.cpp: In function ‘ssize_t utf16_to_utf8_length(const char16_t*, size_t)’:
system/core/libutils/Unicode.cpp:487:78: error: ‘std::basic_ostream<char>::__ostream_type’ has no member named ‘str’
     std::string logged_string = (std::ostringstream() << ret_difference).str();

Bug: 29250543

Test: make winsdk-tools, ran libutils_test, checked that safetynet logs appear using adb


Change-Id: I09fc1e3e0dcc4683bde987f9e23e3bb4bea8b367