fix #1690 track last seen screen in analytics

fix #1690 track last seen screen in analytics 

ref wordpress-mobile/WordPress-iOS#2081
cc @sendhil 

Note the generic way to track the current active activity requires the GET_TASKS permission so, I reused the last_activity preference.
I usually prefer the app to crash when the manifest is misconfigured. But because this won't be used in a super important feature (in this case only for tracking) and because of [this](http://www.engadget.com/2011/05/24/new-cyanogenmod-lets-you-rule-android-app-permissions-with-an-ir/) kind of permission management app, you're right.

Actually, I'll remove this file, I forgot to reverse the first commit in this branch.