Listener

public class RegistrationBackstackChangeListener implements FragmentManager.OnBackStackChangedListener  {

    private final Activity mActivity;

    public RegistrationBackstackChangeListener(Activity activity){
        mActivity = activity;
    }

    @Override
    public void onBackStackChanged() {

    }
}

RegistrationModule

@Module(
        injects = {
                RegistrationActivity.class,
                ApiKeyFragment.class
        }
)
public class RegistrationModule {
    private final Activity mActivity;

    public RegistrationModule(Activity activity){
        mActivity = activity;
    }

    @Provides public FragmentManager.OnBackStackChangedListener provideBackstackChangeListener(){
        return new RegistrationBackstackChangeListener(mActivity);
    }
}

Some Activity

public class SomeActivity extends Activity implements ApiKeyFragment.OnFragmentInteractionListener{

    @Inject FragmentManager.OnBackStackChangedListener mStackChangeListener;
    private ObjectGraph mObjectGraph;

    @Override
    public void onAttachedToWindow() {
        super.onAttachedToWindow();
    }
    
        @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_registration);
        EveSlotsApp app = (EveSlotsApp) getApplication();
        mObjectGraph = app.getObjectGraph().plus(new RegistrationModule(this));
        mObjectGraph.inject(this);
    }
}