My Site Dashboard: Tabs - Track default tab experiment variant assigned

[zwarm]

Parent #15989

This PR tracks MY_SITE_DEFAULT_TAB_EXPERIMENT_VARIANT_ASSIGNED when the value is set. This is needed because the "signed_in" event is called from LoginAnalyticsTracker and we shouldn't be injecting logic for setting the experiment variant in a tracker. Thus we set the variant in LoginEpilogue, but the signed_in events have already been tracked at this point. This event will give us a starting point for funnels following a sign up or login.

To test:
Test A: Login

• Launch the app
• If not enabled, please enable MySiteDashboardTabsFeatureConfig, then restart the app and logout
• Login
• Verify the logs contain: (the default can be dashboard or site_menu)
  🔵 Tracked: my_site_default_tab_experiment_variant_assigned, Properties: {"default_tab_experiment":"dashboard"}

Test B: Signup

• Launch the app
• If not enabled, please enable MySiteDashboardTabsFeatureConfig, then restart the app and logout
• Signup with a new account
• Verify the logs contain: (the default can be dashboard or site_menu)
  🔵 Tracked: my_site_default_tab_experiment_variant_assigned, Properties: {"default_tab_experiment":"dashboard"}

Regression Notes

1. Potential unintended areas of impact
   The event is not tracked

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

3. What automated tests I added (or what prevented me from doing so)
   Added two new tests to MySiteDefaultTabExperimentTest

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.

FYI: @leandroalonso

[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-16187-build-120183.apk

• WordPress-pr-16187-build-120183.apk

[AjeshRPai]

I have looked at the code changes and tested it as per the instructions. Everything looks good to me. @zwarm . 👍 🙌