[Task] Update theme updates to use the approach that guarantees delivery

[chipsnyder]

Fixes #3163
Related PRs:

• Gutenberg: [RNMobile] Update theme updates to use the approach that guarantees delivery WordPress/gutenberg#30847
• iOS: Rework EditorThemeStore to be stored in CoreData WordPress-iOS#16282

📓 This PR can merge as soon as the Gutenberg PR merges but is independent of any of the other changes.

To test:

This can be tested by running the regression test suite:

- [ ] Default Colors - Check that default colors still load - [steps](https://github.com/wordpress-mobile/test-cases/blob/master/test-cases/gutenberg/editor-theme.md#tc001) 
- [ ] Default Gradients - Check that default gradients still load - [steps](https://github.com/wordpress-mobile/test-cases/blob/master/test-cases/gutenberg/editor-theme.md#tc002)
- [ ] Custom Colors - Check that custom colors load in the editor - [steps](https://github.com/wordpress-mobile/test-cases/blob/master/test-cases/gutenberg/editor-theme.md#tc003)
- [ ] Custom Gradients - Check that custom gradients load in the editor - [steps](https://github.com/wordpress-mobile/test-cases/blob/master/test-cases/gutenberg/editor-theme.md#tc004)
- [ ] Offline Support - [steps](https://github.com/wordpress-mobile/test-cases/blob/master/test-cases/gutenberg/editor-theme.md#tc005)

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered if this change warrants user-facing release notes more info and have added them to RELEASE-NOTES.txt if necessary.

[chipsnyder]

@dcalhoun @enejb CC'ing you two since you're the release wranglers for this release. This isn't required for 1.51 but I would like to get it merged for that release if possible. Added you as reviewers just so you're aware.

[antonis]

Approved via WordPress/gutenberg#30847 🎉

[peril-wordpress-mobile]

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