protected DataSource<Object> getAndSubscribe() {
      DataSource<Object> dataSource = mDataSourceSupplier.get();
      dataSource.subscribe(mDataSubscriber, mExecutor);
      return dataSource;
    }
Gets data source from our mDataSourceSupplier and subscribes mDataSubscriber to it. Obtained
data source is returned.
fbcore/src/test/java/com/facebook/datasource/DataSourceTestUtils.java