* a11y: implement new SemanticsAction "showOnScreen" (v2)

This action is triggered when the user swipes (in accessibility mode) to the last visible item of a scrollable list to bring that item fully on screen.

This requires engine rolled to flutter/engine#3856.

I am in the process of adding tests, but I'd like to get early feedback to see if this approach is OK.

* fix null check

* review comments

* review comments

* Add test

* fix analyzer warning