[image_picker] Move I/O operations to a separate thread (#3506)

Many I/O operations in the `image_picker` are currently carried out on the main thread (at least on Android). This blocks the UI (main) thread, freezing the UI and sometimes even causing an ANR dialog to pop up.

With these PR, many of said I/O operations now run on a separate thread. Specifically, it executes all code that is run in response to a picking result on a separate thread. So when an image is picked, for example, the callback will be ran outside of the UI thread. This change was rather easy, as said code was already making use of async paradigms.

The I/O operations carried out for caching logic (`ImagePickerDelegate.retrieveLostImage()`) are **not** changed by this PR, as my guess is that this would call for breaking changes to the API, as the result is currently returned synchronously instead.

When running the example app in [StrictMode](https://developer.android.com/reference/android/os/StrictMode) and picking an image, the problem becomes apparent, as it reports disk read/write violations. This was described in [#100966](https://github.com/flutter/flutter/issues/100966). I have verified that StrictMode no longer reports disk read/write violations after the changes. To check the output of StrictMode, I ran the example app with the changes listed [here](https://github.com/math1man/plugins/commit/11f5f8ace392d061b442d38e1237655bd5628f0f).

This PR closes several issues related to ANRs.

* Fixes [#94120](https://github.com/flutter/flutter/issues/94210)
* Fixes [#114080](https://github.com/flutter/flutter/issues/114080)

Also, it partly implements what is requested in [#91393](https://github.com/flutter/flutter/issues/91393).
4 files changed
tree: 9f55a05c35c75223a94b7242783cca5f5ce3bbb4
  1. .ci/
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  3. packages/
  4. script/
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  6. .ci.yaml
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  10. .gitignore
  11. .gitmodules
  12. .metadata
  13. analysis_options.yaml
  14. AUTHORS
  15. CODEOWNERS
  16. CONTRIBUTING.md
  17. customer_testing.bat
  18. customer_testing.sh
  19. LICENSE
  20. README.md
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