[cp][ios] Link PlatformView back to semantics tree (#46471) (#46900)

Cherry pick https://github.com/flutter/engine/commit/250daadbca517677317277f314ea2a40568eb345

Cherry pick issue https://github.com/flutter/flutter/issues/136552

Description:

The PlatformView does not have a semantics container when added to semantics tree, this PR gives it a semantics container to ensure accessibility traversal works.

This fixes https://github.com/flutter/flutter/issues/135504, which is a regression of https://github.com/flutter/engine/commit/b8332e3c4870512a941d685d7d375b1d2d62983c

Before https://github.com/flutter/engine/commit/b8332e3c4870512a941d685d7d375b1d2d62983c, the traversal works because the PlatformView is added to the accessibilityElements of the FlutterPlatFormViewSemanticsContainer, which implicitly made the FlutterPlatFormViewSemanticsContainer as the PlatformVIew's AccessibilityContainer. 

Now we use the PlatformVIew as the nativeAccessibility of the  FlutterPlatFormViewSemanticsContainer, we need to expilicitly set the container. 

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
12 files changed
tree: 1639cd9578d93df77c62924d276b005cef9c15d2
  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. CODEOWNERS
  36. CONTRIBUTING.md
  37. DEPS
  38. Doxyfile
  39. LICENSE
  40. README.md
README.md

Flutter Engine

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