Fix Classic Block in the Unsupported Block editor

[cameronvoell]

Description from (#12679), cherry-picked commit into the new gutenberg/fix-classic-block-in-unsupported-editor-15.4.1-base branch with proper hotfix base (release/15.4.1).

This PR introduces a fix for the block editor for better support of the Classic Block.

Gutenberg-mobile PR: wordpress-mobile/gutenberg-mobile#2550

It's based on gb-mobile v1.33.1.

To test:

1. Switch to html mode and write some text in-between two blocks, making sure the text is not inside the block delimiters
2. Switch back to visual mode
3. There should be an unsupported block displayed between the blocks the text was inserted
4. Use the "?" button of the unsupported block and then "Edit block in web browser" to launch the Unsupported Block editor
5. Notice the text being displayed inside a "Classic" block without an error 🎉
6. Make some changes to the text and hit the "✔️" icon in the upper right to commit the changes
7. Switch to html mode again and notice that the text includes the changes made inside the Unsupported Block editor 🎉

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

	Warnings
⚠️	PR is not assigned to a milestone.

Generated by 🚫 dangerJS

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading the APK here.

[marecar3]

LGTM! Working as expected!