Stats: Refs #874. Retains the selection under Visitors and Views tab when rotated.

The selection wasn't being saved and persisted when the fragment was destroyed and re-created.
Merge branch 'develop' of https://github.com/wordpress-mobile/WordPress-Android into develop
Merge branch 'develop' of https://github.com/wordpress-mobile/WordPress-Android into develop
Stats: Refs #874. Using setRetainInstance(true) retains the selection under Visitors and Views tab, when rotated.
Adding WPStats and WPStatsTrackerMixpanel.

WPStats will replace WPMobileStatsUtil as the new class to handle all
stats related calls in the Android app.
Adding some initial code to mixpanel tracker.
Added WPStats endpoints for themes and reader.
Disabling WPMobileStatsUtil's mixpanel calls for now.
Removing WPMobileStatsUtil unused event as it's a part of WPStats.
Adding WPStats tracking for stats.
Fixing crash in WPStatsTrackerMixpanel
Adding initial WPStats for editor related events.
Adding more WPStats for editor related events.
Adding WPStats events for notifications.
Adding WPStats for tracking the opening of various activities.
Adding WPStats calls for creating account and site.
Fleshing out WPStatsTrackerMixpanel.

Added in code to allow WPStatsTrackerMixpanel to perform actions on
the other possible things we might want to send to mixpanel (i.e.
people properties, super properties, etc.).
Adding WPStats tracking for view admin.
Clearing out mixpanel data on app end and logout.
Fixed bug where user's properties weren't being included in stats.