Added UserPrefs methods to get/set/reset the last activity string, now read at startup when restoring the previously selected activity
Replaced all usage of commit() with apply() in UserPrefs, as per http://developer.android.com/reference/android/content/SharedPreferences.Editor.html#apply%28%29
Added PREFKEY_LAST_ACTIVITY_STR to UserPrefs.reset()
Merge pull request #1741 from wordpress-mobile/issue/1729-restore-last-activity

Issue/1729 restore last activity