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
Rather than read/write this pref here, can we just add it to UserPrefs.java? It would be best, I think, if we centralized all the prefs we use, especially ones like this which need to be reset on logout.