Interactivity API: Fix state intialization for asynchronous private stores

[DAreRodz]

What?

Fixes a bug that appears when a private store is loaded asynchronously, and the page contains the initial state for that store.

Why?

The current implementation makes those stores become public, thus making private stores throw an error when they try to lock their namespace.

How?

Using the universalUnlock variable defined in store.ts. That prevents the store from being marked as "public" and can be set as "private" later on.

[github-actions]

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Core SVN

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Props darerodz, luisherranz.

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Co-authored-by: DAreRodz <darerodz@git.wordpress.org>
Co-authored-by: luisherranz <luisherranz@git.wordpress.org>

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[luisherranz]

LGTM!