fix #1689 use me endpoint email mixpanel

I copied the behavior of https://github.com/wordpress-mobile/WordPress-iOS/pull/2080 
@sendhil can you confirm this comment is correct: https://github.com/wordpress-mobile/WordPress-Android/blob/f0a1f1ed7faede1064596991440a37cf001a1dd2/WordPress/src/main/java/org/wordpress/android/analytics/AnalyticsTrackerMixpanel.java#L85
I was thinking of doing that, but since it's not a user "preference", I kept it here. I think I misunderstood what UserPrefs was used for ;). I'll move it there.