[YIR] Assign logged-in users to YiR experiment group

[tonisevener]

Phabricator: https://phabricator.wikimedia.org/T406947

Notes

When evaluating data, we discovered group assignments were missing for logged-in users. Even though the app's behavior won't change for them (both groups will not see a login prompt because they are logged in, and both groups will be eligible for personalized slides), we still want a group assignment for these users for retention comparisons on the backend.

Test Steps

1. Set WMFLogging.h = .all
2. Fresh install, select EN Wikipedia as primary app language.
3. Cancel out of any YiR feature announcement.
4. Log in to app, build up a history of several articles.
5. Go to Year in Review from Profile. Tap Get Started. In the console you should now see a group assignment for the start_click action data.
6. You should see personalized slides, no login prompt after tapping Get Started.
7. Repeat steps 2 - 6, but this time choose a non-EN primary app language. In step 5, you should continue to NOT see a group assignment when sending the start_click event.