[camera_android_camerax] Fix leaked thread per capture in takePicture

[toyaji]

Fixes flutter/flutter#190523

Problem

ImageCaptureProxyApi.takePicture creates a new single-thread executor on
every call and never shuts it down. The thread it owns is only reclaimed
once its FinalizableDelegatedExecutorService wrapper is finalized, at a
time the plugin does not control.

That executor only delivers an already finished result — it does no real
work:

• Encoding and file I/O run on CameraX's own IO executor.
• TakePictureRequest.onResult does nothing but hand the result to the
  executor the plugin passed in.

Fix

Dispatch the result on the main executor instead, which is what this
plugin's other CameraX callbacks already use:

• CameraControlProxyApi
• Camera2CameraControlProxyApi
• ImageAnalysisProxyApi
• ProcessCameraProviderProxyApi
• PendingRecordingProxyApi

Note

PreviewProxyApi uses the same per-request executor pattern for
SurfaceRequest.provideSurface. That callback releases a Surface, so it
is left for a separate change.

The two existing takePicture tests now stub the main executor on their
mock Context.

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under.
• All existing and new tests are passing.

[gemini-code-assist]

Code Review

This pull request addresses a thread leak in the camera_android_camerax package by dispatching the takePicture result callback on the main executor instead of a new single-thread executor. Unit tests in ImageCaptureTest.java have been added and updated to verify this change. There are no review comments, so I have no feedback to provide.

[camsim99]

Thanks for filing a clear issue and the quick fix! This totally makes sense to me.

[flutter-dashboard]

This pull request is not mergeable in its current state, likely because of a merge conflict. Pre-submit CI jobs were not triggered. Pushing a new commit to this branch that resolves the issue will result in pre-submit jobs being scheduled.

[auto-submit]

autosubmit label was removed for flutter/packages/12361, because Pull request flutter/packages/12361 is not in a mergeable state.

[toyaji]

@camsim99 @gmackall Rebased to resolve the CHANGELOG conflict — version is now 0.7.4+5 since #12145 took +4. No code changes. Ready for the CICD and autosubmit labels again.