* Extract WideToUTF16String/UTF16StringToWide to FML
In third_party/accessibility, for string conversion, we use a mix of:
* FML
* third_party/accessibility base string utility functions
* static functions local to the translation unit itself
This moves all conversions between UTF16 and wide strings to FML. Note
that this implementation is only safe on platforms where:
* the size of wchar_t and char16_t are the same
* the encoding of wchar_t and char16_t are both UTF-16
which is the case for Windows, hence why these functions are implemented
in a Windows-specific translation unit (wstring_conversion).
Issue: https://github.com/flutter/flutter/issues/118811
* Migrate UTF16ToWide as well
d96d942ddc