[Woo POS] don't set loading state for pull to refresh

[joshheald]

Closes: #14869
Merge after: #15077

Description

This PR stops us adding a ghost/skeleton cell to the bottom of the item list when we pull to refresh.

When we pull to refresh, the built in controller shows a loading indicator. Previously, we set our state to loading, so we also showed a ghost cell at the bottom of the list.

On a pull to refresh, it’s unlikely that the list is going to get longer – more likely, the full first page of results will be shown, so it’ll be the same length or shorter.

Previously we had to put our refreshable modifiers higher in the heirarchy than the views they were refreshing, because we changed the state to loading when we pulled to refresh. This change of state cancelled the pull to refresh network task. Now we can group the refreshable control more closely with the list view it relates to.

Steps to reproduce

This is easiest to test with a small page size, since you can see whether there's a ghost cell on PTR without scrolling down, then triggering the load next page and (correctly) showing a ghost cell for that. However, if the items don't fill the screen the next page load won't trigger. You can use the dynamic type to get around that.

1. Launch the app and open POS
2. Pull to refresh
3. Observe that the ghost cell is not shown
4. Load more pages
5. Pull to refresh
6. Observe that the refresh worked as expected.

Testing information

Screenshots

pull.to.refresh.without.ghosts.mp4

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[wpmobilebot]

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

	App Name	WooCommerce iOS
	Build Number	pr15079-2c19458
	Version	21.6
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	2c19458
	App Center Build	WooCommerce - Prototype Builds #12859

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[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 21.7. This milestone is due in less than 2 days. Please make sure to get it merged by then or assign it to a milestone with a later deadline.

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[jaclync]

Thank you for making this change! This has been bothering me mildly, as this happens easily for variation list with few variations. 🚀