Stabilize Virtualize initial item positioning (#68886) (#69170)

* Stabilize Virtualize initial item positioning.

* Rename.

* Boarder should not take part in calculations.

* Improve unit test readibility.

* Treat unavailable final InitialItemIndex alignment as aborted.

* Preserve Virtualize running item size average. Use the accumulated measurement when applying initial-index geometry instead of replacing it with the latest rendered window, and add regression coverage for the measurement history.

* Unquarantine.

Co-authored-by: Ilona Tomkowicz <32700855+ilonatommy@users.noreply.github.com>