Fix Core Data concurrency issues in AccountService

[crazytonyli]

I noticed some Core Data concurrency issues on accessing WPAccount from a wrong thread.

I don't see a huge risk associated with these changes. The updateDefaultBlogIfNeeded function change is only a signature change, the code is still executed the way it used to be. The real logic of setupAppExtensionsWithDefaultAccount is now forced to run in the main thread, which is where it used to run too.

Test instructions

A couple of features to verify:

• Log in and log out of the app using a WP.com account.
• "Save as Draft" share extension (i.e. from Safari).

Regression Notes

1. Potential unintended areas of impact
   None.

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   What's described in the "Test instructions"

3. What automated tests I added (or what prevented me from doing so)
   None.

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• 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.

[wpmobilebot]

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

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr20394-b1a6ae8
	Version	22.0
	Bundle ID	com.jetpack.alpha
	Commit	b1a6ae8
	App Center Build	jetpack-installable-builds #4338

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[wpmobilebot]

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

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr20394-b1a6ae8
	Version	22.0
	Bundle ID	org.wordpress.alpha
	Commit	b1a6ae8
	App Center Build	WPiOS - One-Offs #5312

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.