Log out: Ensure that unregistering push notifications completes

[itsmeichigo]

Part of WOOMOB-1755

Description

We got reports about some devices still get push notifications even after logging out. While testing, I notice that very often I don't see 📱 Successfully unregistered from WordPress.com Push Notifications! in my logs after logging out.

This PR adds updates to ensure that push notifications are unregistered completely upon logout. The solution is to keep a strong reference of the authenticated state until unregisteration is finished.

Test Steps

• Build and run the app on a device.
• Log in to a test store with a WPCom account if you haven't already.
• Log out of the app.
• Tap Log in > Help > View application Log > Current.
• Scroll to the bottom of the log.
• Confirm that you see 📱 Successfully unregistered from WordPress.com Push Notifications! near the bottom.

Screenshots

N/A

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• 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 WooCommerce iOS Prototype by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce iOS Prototype
	Build Number	pr16386-5f51a1d
	Version	23.7
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	5f51a1d
	Installation URL	508k4h2gqa6kg

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

[RafaelKayumov]

Works as described.