Workaround for double Jetpack prompt

I think there's a deeper problem in the way `initStats` can get called
recursively, but this works as a quick fix for 4.2

Fixes #1959
Use a property instead of ivar.
Merge branch 'release/4.2' into issue/1959-jetpack-login

Conflicts:
	WordPress/Classes/ViewRelated/Stats/StatsViewController.m
Merge pull request #1963 from wordpress-mobile/issue/1959-jetpack-login

Workaround for double Jetpack prompt