Refactor the way locally saving an edited custom field

[hafizrahman]

Closes: #

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[dangermattic]

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📲 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	pr14098-b21b6e8
	Version	20.5
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	b21b6e8
	App Center Build	WooCommerce - Prototype Builds #11103

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[hafizrahman]

Closing as this was just a POC for discussion.