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.
For safety, I think this needs to catch IllegalArgumentException (in case a bogus/outdated activity name is in prefs).