[Android] Made DirectoryActivity extend a ListActivity, since that's basically what its purpose is - to act as a list view.
[Android] Made it unnecessary for HistorySelection.java, CoreSelection.java, and DirectoryActivity.java to implement an AdapterView. Since these now extend ListActivity, they can just override the method onListItemClick().
Merge pull request #330 from lioncash/master

More simplifications and API 'correctness' for Android.