Call AVCaptureSession startRunning from a background thread

[mokagio]

While working on #19635, Xcode reported:

Thread Performance Checker: -[AVCaptureSession startRunning] should be called from background thread. Calling it on the main thread can lead to UI unresponsiveness

Testing

I applied the code and run through the Scan QR code flow (visit this page on the desktop browser) and verified the TSAN warning was gone.

As an aside, I've been getting the following error, but I'm 99.9% sure it's unrelated to the code change: Update The reason was that I was using an A8c account, for which QR code login doesn't work.

Error screenshot

Regression Notes

1. Potential unintended areas of impact – N.A.
2. What I did to test those areas of impact (or what existing automated tests I relied on) – N.A.
3. What automated tests I added (or what prevented me from doing so) – N.A.

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

[crazytonyli]

I think we should use a serial queue to start and stop the camera session. See this Apple sample project.

[wpmobilebot]

You can test the changes in Jetpack from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr19636-09f44d8 on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[wpmobilebot]

You can test the changes in WordPress from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr19636-09f44d8 on your iPhone

If you need access to App Center, please ask a maintainer to add you.