Start QS only for the newly created site

[planarvoid]

Fixes #14178

This PR removes the double call to the passActivityResultToMySiteFragment. In addition instead of starting the QS for the selected site, I'm passing the created site ID to the function. I think this approach should be much less error prone. Let me know what you think!

To test:

• Make sure you're on a freshly installed app with a site
• Turn on the My Site Improvements flag and restart the app
• Create a site
• Notice the new site has QS started
• Switch site to the previous site
• Notice the previous site doesn't have QS started

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]

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

[renanferrari]

Thanks for looking into this and for coming up with a solution! The code is looking good and I've tested the app with the feature flag both enabled and disabled and everything seems to be working as expected.

LGTM