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().
Maybe it could be used outside of the class at some point, so might be better to leave constructors public in general.