fix: [Expandable] Correct icon color

[meissadia]

Closes #385

Changes

• Match structure of DS Expandable component to allow the existing CSS styles for icon color to be correctly applied to the DSR Expandable

How to test this PR

1. Compare screenshots
2. Compare live examples
   • yarn start
   • Visit http://localhost:6006/?path=/docs/components-draft-expandable-single--overview
   • Compare to https://cfpb.github.io/design-system/components/expandables

Screenshots

Before

After

DS Expandable

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