[CP-beta][Android] post image task to main handler. (#52166)

This pull request is created by [automatic cherry pick workflow](https://github.com/flutter/flutter/wiki/Flutter-Cherrypick-Process#automatically-creates-a-cherry-pick-request)
Please fill in the form below, and a flutter domain expert will evaluate this cherry pick request.

### Issue Link:
What is the link to the issue this cherry-pick is addressing?

Test failures in Impeller migration PR: https://github.com/flutter/packages/pull/6461

### Changelog Description:
Explain this cherry pick in one line that is accessible to most Flutter developers. See [best practices](https://github.com/flutter/flutter/wiki/Hotfix-Documentation-Best-Practices) for examples

Ensure that Android platform view updates are posted on correct thread when platform view is created on background handler.

### Impact Description:
What is the impact (ex. visual jank on Samsung phones, app crash, cannot ship an iOS app)? Does it impact development (ex. flutter doctor crashes when Android Studio is installed), or the shipping production app (the app crashes on launch)

Migration to Impeller plugin APIs is blocked.

### Workaround:
Is there a workaround for this issue?

Disable impeller

### Risk:
What is the risk level of this cherry-pick?

### Test Coverage:
Are you confident that your fix is well-tested by automated tests?

  However the tests are in flutter/packages and not flutter/engine.

### Validation Steps:
What are the steps to validate that this fix works?

Run the unit tests on the linked PR in flutter/packages
1 file changed
tree: 39e4374a78c7b20b2d79755c8e28732ee70b83e2
  1. .github/
  2. assets/
  3. benchmarking/
  4. bin/
  5. build/
  6. build_overrides/
  7. ci/
  8. common/
  9. display_list/
  10. docs/
  11. examples/
  12. flow/
  13. flutter_frontend_server/
  14. flutter_vma/
  15. fml/
  16. impeller/
  17. lib/
  18. runtime/
  19. shell/
  20. skia/
  21. sky/
  22. testing/
  23. third_party/
  24. tools/
  25. vulkan/
  26. wasm/
  27. web_sdk/
  28. .ci.yaml
  29. .clang-format
  30. .clang-tidy
  31. .gitattributes
  32. .gitignore
  33. .pylintrc
  34. .style.yapf
  35. analysis_options.yaml
  36. AUTHORS
  37. BUILD.gn
  38. CODEOWNERS
  39. CONTRIBUTING.md
  40. DEPS
  41. Doxyfile
  42. LICENSE
  43. README.md
README.md

Flutter Engine

Flutter CI Status OpenSSF Scorecard SLSA 1

Flutter is Google's SDK for crafting beautiful, fast user experiences for mobile, web, and desktop from a single codebase. Flutter works with existing code, is used by developers and organizations around the world, and is free and open source.

The Flutter Engine is a portable runtime for hosting Flutter applications. It implements Flutter's core libraries, including animation and graphics, file and network I/O, accessibility support, plugin architecture, and a Dart runtime and compile toolchain. Most developers will interact with Flutter via the Flutter Framework, which provides a modern, reactive framework, and a rich set of platform, layout and foundation widgets.

If you want to run/contribute to Flutter Web engine, more tooling can be found at felt. This is a tool written to make web engine development experience easy.

If you are new to Flutter, then you will find more general information on the Flutter project, including tutorials and samples, on our Web site at Flutter.dev. For specific information about Flutter's APIs, consider our API reference which can be found at the docs.flutter.dev.

Flutter is a fully open source project, and we welcome contributions. Information on how to get started can be found at our contributor guide.