onPostExecute stress fix + settings strings tweak

Added extra null checks to onPostExecute methods in ASynkTasks to prevent crashes when under stress, as noted [here](https://github.com/the-blue-alliance/the-blue-alliance-android/issues/42)

Also includes the string hardcode to Strings resources fixes from [my previous pull request](https://github.com/the-blue-alliance/the-blue-alliance-android/pull/33)