Move the code snippet which caused ANR to background thread

[jarvislin]

Fixes #20832

This is an ANR issue, we fetched data from database in main thread in the class of site header.

This PR moves the code snippet which caused ANR to background thread by using ExecutorService, and then handles the UI related code in main thread with Handler.

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To Test:

1. Sign in JP app.
2. Switch to the Notifications tab.
3. Find a comment type notification and click on it.
4. Click on user's avatar.
5. It will navigate you to ReaderPostList.
6. It should display the site header correctly.
7. Done, thank you!

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Regression Notes

1. Potential unintended areas of impact

   • reader

2. What I did to test those areas of impact (or what existing automated tests I relied on)

   • manual

3. What automated tests I added (or what prevented me from doing so)

   • n/a

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PR Submission Checklist:

• I have completed the Regression Notes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

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Testing Checklist (strike-out the not-applying and unnecessary ones):

Skipped

[sonarcloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Flavor	Jalapeno
	Build Type	Debug
	Version	pr20833-03a7cbd
	Commit	03a7cbd
	Direct Download	jetpack-prototype-build-pr20833-03a7cbd.apk

Note: Google Login is not supported on these builds.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress by scanning the QR code below to install the corresponding build.

	App Name	WordPress
	Flavor	Jalapeno
	Build Type	Debug
	Version	pr20833-03a7cbd
	Commit	03a7cbd
	Direct Download	wordpress-prototype-build-pr20833-03a7cbd.apk

Note: Google Login is not supported on these builds.

[justtwago]

Thanks for the fix Jarvis! Works as expected 🚢