Refactor MainActivity adapter setup

[tobrun]

We currently have a MainActivity#listItems method that generates all the required adapter items. Currently everything is hard coded in one method block. We should look into generating code for this instead.

[langsmith]

@tobrun , how do you envision improving this? MainActivity for reference

Anything else besides breaking things up into separate methods (seen below)? Or would that be enough/fine?

private void buildPlugins() {
    exampleItemModel.add(new ExampleItemModel(
      R.string.activity_plugins_traffic_plugin_title,
      R.string.activity_plugins_traffic_plugin_description,
      new Intent(MainActivity.this, TrafficPluginActivity.class),
      R.string.activity_plugins_traffic_plugin_url
    ));
    exampleItemModel.add(new ExampleItemModel(
      R.string.activity_plugins_building_plugin_title,
      R.string.activity_plugins_building_plugin_description,
      new Intent(MainActivity.this, BuildingPluginActivity.class),
      R.string.activity_plugins_building_plugin_url, true
    ));
    exampleItemModel.add(new ExampleItemModel(
      R.string.activity_plugins_location_plugin_title,
      R.string.activity_plugins_location_plugin_description,
      new Intent(MainActivity.this, LocationPluginActivity.class),
      R.string.activity_plugins_location_plugin_url, true
    ));
  }

and then in the switch/case statement 👇

case R.id.nav_plugins:
        buildPlugins();
        currentCategory = R.id.nav_plugins;
        break;

Do the same for all of the nav drawer example sections.