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 am not sure why...I would leave public if not needed. Actually I am even thinking that we could remove this constructor and just do new Stats(stats.queryCount etc.) directly, which would address my other comment too