Indices API: Fix wrong search stats groups

This provides a fix to issue #7644. A new Stats object must be created, and not a reference to the retrieved stats, before we can add stats to it. Otherwise, we would keep on adding to the same object on subsequent calls to IndicesStatsResponse#getPrimaries() or IndicesStatsResponse#getTotal().
I know the result is the same, but we are not adding to something existing, we are creating a new object. The addition is not needed and I think it would make the code cleaner if we called the other constructor.