Push notifications: Clean up logic for register/unregister tokens

[itsmeichigo]

Closes WOOMOB-1851

Description

In a past project we added the support the register a single token to get push notifications for all stores. However, in the current codebase we still have some confusing logic:

• Current site ID is sent to the network layer even though it's no longer needed to register PN tokens.
• Unregistration requests are made when switching stores even though the same token is used for all stores.

This PR cleans up the logic:

• Removes the redundant selected_blog_id in the request and the related param in methods to avoid confusion.
• Removes the unregistration request when switching stores.

I'm keeping the registration request when switching stores for simplicity. It's recommended by Apple to re-register often in case tokens are invalidated remotely by Apple, and WPCom server can disregard the request if an existing token is re-registered.

Test Steps

• Log in to a test store with a WPCom account that is connected to more than 1 stores in a physical device.
• Put your device to sleep.
• Place an order on the store that you're currently selecting on the app (let's call this store A). Confirm that you get notification about the new order.
• Open the app and switch to another store (store B) and put the device back to sleep.
• Place an order on store B. Confirm that you get notification for this order.
• Placer another order on store A. Confirm that you get notification for this order.

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	pr16434-e591de3
	Version	23.8
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	e591de3
	Installation URL	6lems67uf3sdo

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

[adborbas]

@itsmeichigo, how can I enable push notifications in the PR build? I seem only to receive them in the store build.

[itsmeichigo]

how can I enable push notifications in the PR build? I seem only to receive them in the store build.

@adborbas I don't think the alpha build supports notifications 🤔 Can you build the app to your device in debug mode? Debug build supports notifications.

[adborbas]

Nice, work as described!