[Home Page Picker] Mark Theme Selection as Completed as part of the Site Creation Flow

[antonis]

With the release of the home page picker #13409
We make users pick a theme during site creation. We thus need to mark the theme selection step as completed in the quick start tour screen.

To test:

Choose a theme

• Create a site
• Pick a theme
• Take the tour
• Notice "Choose a theme" is marked as completed

Skip site creation

• Create a site
• Select Skip on Choose a Design
• Take the tour
• Notice "Choose a theme" is in the checklist

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.

[antonis]

Closing in favor of #13503 (changed target)

[chipsnyder]

LGTM! Tested on the Skip and choose a design flows 👍