Guard mutex unlock in isLocked check to avoid errors during local data uploads

[ovitrif]

Fixes #17463

The goal of this fix is to prevent possible data loss while their content is uploaded to the server.
While we weren't able to reproduce the crash we noticed it occurs for some users when they start the app, if their internet connectivity is active and there are local changes to be synced to the server.

This fix is applied to to prevent possible IllegalStateExceptions when the mutex is already unlocked by the system during a cancellation / completion while the mutex locking function is suspended.

To test:
Nothing much to test here, verify CI checks are green.

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

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.

[peril-wordpress-mobile]

	Warnings
⚠️	This PR is assigned to a milestone which is closing in less than 4 days Please, make sure to get it merged by then or assign it to a later expiring milestone

Generated by 🚫 dangerJS

[ParaskP7]

LGTM! 💯

Thank you so much for working on making this more robust @ovitrif ! 🙇 ❤️ 🥇

[ovitrif]

Thank you so much for working on making this more robust @ovitrif ! 🙇 ❤️ 🥇

Many thanks for helping with the investigation and the PR review @ParaskP7 🥇 🚀

[wpmobilebot]

📲 You can test these changes on WordPress by downloading wordpress-installable-build-pr17472-749de37.apk

	💡 Scan this QR code with your Android phone to download and install the APK directly on it.
	App	WordPress
	Build Flavor	Jalapeno
	Build Type	Debug
	Commit	749de37

Note: This installable build uses the JalapenoDebug build flavor, and does not support Google Login.

[wpmobilebot]

📲 You can test these changes on Jetpack by downloading jetpack-installable-build-pr17472-749de37.apk

	💡 Scan this QR code with your Android phone to download and install the APK directly on it.
	App	Jetpack
	Build Flavor	Jalapeno
	Build Type	Debug
	Commit	749de37

Note: This installable build uses the JalapenoDebug build flavor, and does not support Google Login.

[AliSoftware]

This fix has been integrated as part of the new 21.2-rc-2 beta 🎉