Android Studio : Implementing Expandable RecyclerView in Master/Detail flow Activity chosen from IDE Activity Selection

Question

I want to manipulate the custom Master/Detail flow given by Android Studio to include an Expandable RecyclerView in place of the simple RecyclerView given in the auto generated activity. Here is the flow I want:

The code below is the activity that handles the RecyclerView:

public class ItemListActivity extends AppCompatActivity {

     /**
     * Whether or not the activity is in two-pane mode, i.e. running on a tablet
     * device.
     */
    private boolean mTwoPane;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_item_list);

        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        toolbar.setTitle(getTitle());

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });

        View recyclerView = findViewById(R.id.item_list);
        assert recyclerView != null;
        setupRecyclerView((RecyclerView) recyclerView);

        if (findViewById(R.id.item_detail_container) != null) {
            // The detail container view will be present only in the
            // large-screen layouts (res/values-w900dp).
            // If this view is present, then the
            // activity should be in two-pane mode.
            mTwoPane = true;
        }
    }

    private void setupRecyclerView(@NonNull RecyclerView recyclerView) {
        recyclerView.setAdapter(new SimpleItemRecyclerViewAdapter(DummyContent.ITEMS));
    }

    public class SimpleItemRecyclerViewAdapter
            extends RecyclerView.Adapter<SimpleItemRecyclerViewAdapter.ViewHolder> {

        private final List<DummyContent.DummyItem> mValues;

        public SimpleItemRecyclerViewAdapter(List<DummyContent.DummyItem> items) {
            mValues = items;
        }

        @Override
        public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
            View view = LayoutInflater.from(parent.getContext())
                    .inflate(R.layout.item_list_content, parent, false);
            return new ViewHolder(view);
        }

        @Override
        public void onBindViewHolder(final ViewHolder holder, int position) {
            holder.mItem = mValues.get(position);
            holder.mIdView.setText(mValues.get(position).id);
            holder.mContentView.setText(mValues.get(position).content);

            holder.mView.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    if (mTwoPane) {
                        Bundle arguments = new Bundle();
                        arguments.putString(ItemDetailFragment.ARG_ITEM_ID, holder.mItem.id);
                        ItemDetailFragment fragment = new ItemDetailFragment();
                        fragment.setArguments(arguments);
                        getSupportFragmentManager().beginTransaction()
                                .replace(R.id.item_detail_container, fragment)
                                .commit();
                    } else {
                        Context context = v.getContext();
                        Intent intent = new Intent(context, ItemDetailActivity.class);
                        intent.putExtra(ItemDetailFragment.ARG_ITEM_ID, holder.mItem.id);

                        context.startActivity(intent);
                    }
                }
            });
        }

        @Override
        public int getItemCount() {
            return mValues.size();
        }

        public class ViewHolder extends RecyclerView.ViewHolder {
            public final View mView;
            public final TextView mIdView;
            public final TextView mContentView;
            public DummyContent.DummyItem mItem;

            public ViewHolder(View view) {
                super(view);
                mView = view;
                mIdView = (TextView) view.findViewById(R.id.id);
                mContentView = (TextView) view.findViewById(R.id.content);
            }

            @Override
            public String toString() {
                return super.toString() + " '" + mContentView.getText() + "'";
            }
        }
    }
}