Track Mixpanel analytics event "stats scrolling to bottom"

Fix #1714 by tracking the scrolling state of the main ScrollView used in Stats screen.

Unfortunately there is not an obvious way to do this without extending ScrollView.
also used a limited task with a guard of 2 secs, otherwise multiple bottom reached events can be tracked at the same time.

/cc @sendhil 
A simpler way to do this is to use `canScrollVertically()`, like this:
```
boolean canScrollDown = scrollView.canScrollVertically(1);
if (!canScrollDown) {
    sTrackBottomReachedStats.runIfNotLimited();
}
```

I've been using this reliably in [WPListView](https://github.com/wordpress-mobile/WordPress-Android/blob/develop/WordPress/src/main/java/org/wordpress/android/widgets/WPListView.java) for a while.