Move custom indicator to be rendered above the colors

[lukewalczak]

Fixes: #3097

To test:

1. Go to a post/page.
2. Add Buttons block.
3. Tap on the gear button.
4. Tap on Background Color options.
5. Switch several times between Solid and Gradient segments.
   Expect app mustn't crash

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.

[fluiddot]

Tested and approved via WordPress/gutenberg#28698 (review).

[fluiddot]

@lukewalczak I think this PR is ready to be merged, although first we should resolve the conflict in the gutenberg submodule reference.

[peril-wordpress-mobile]

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