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.
This routine isn't used anywhere. If we're going to keep it for possible future use, we should catch SecurityException when calling getRunningTasks() in the case the app doesn't have GET_TASKS permission.