[Site design revamp] Remove selection border when collapsing themes

[antonis]

Removes the pink selection border when the preview is collapsing (ref: pc8HXX-oi-p2#comment-503)

To test:

1. Start the site creation flow
2. Proceed to the design picker
3. Select a layout
4. slowly dismiss the preview
5. Verify that no selection border is visible (like below)
   

Regression Notes

1. Potential unintended areas of impact
   Page Layout picker

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   Manual testing

3. What automated tests I added (or what prevented me from doing so)
   The fix was in the UI layer that would be hard to test with the current code structure

PR submission checklist:

• I have completed the Regression Notes.
• 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]

You can test the changes on this Pull Request by downloading the APKs:

• Jetpack-pr-16693-build-124718.apk

• WordPress-pr-16693-build-124718.apk

[twstokes]

I tested Jetpack-pr-16693-build-124718.apk and it works as described. 🥳

I also tested the Page Layout picker and didn't spot any regressions. Thanks @antonis!