Extract WideToUTF16String/UTF16StringToWide to FML (#39006)

* 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
10 files changed
tree: 7e0e7c7f2a0cc4ddf9b2448de0537e124280edc6
  1. .github/
  2. assets/
  3. benchmarking/
  4. build/
  5. ci/
  6. common/
  7. display_list/
  8. docs/
  9. examples/
  10. flow/
  11. flutter_frontend_server/
  12. flutter_vma/
  13. fml/
  14. impeller/
  15. lib/
  16. runtime/
  17. shell/
  18. sky/
  19. testing/
  20. third_party/
  21. tools/
  22. vulkan/
  23. wasm/
  24. web_sdk/
  25. .ci.yaml
  26. .clang-format
  27. .clang-tidy
  28. .gitattributes
  29. .gitignore
  30. .pylintrc
  31. .style.yapf
  32. analysis_options.yaml
  33. AUTHORS
  34. BUILD.gn
  35. CONTRIBUTING.md
  36. DEPS
  37. Doxyfile
  38. LICENSE
  39. README.md
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