Abstract out aria-activedescendant handling

Description

Abstract out the implementation of aria-activedescendant handling implemented in gitlab-org/gitlab-ui#3159 (closed) to allow simplified re-use in a number of UI components.

The aria-activedescendant property provides a method of managing focus for assistive technologies on interactive elements when they contain multiple focusable descendants, such as menus, grids, and toolbars. Instead of the screen reader moving focus between owned elements, aria-activedescendant can be used on container elements to refer to the currently active element, informing assistive technology users of the currently active element when focused.

Full aria spec here.

Why?

Before aria-activedescendant	After aria-activedescendant
Focus leaves the input to navigate to an option. The user cannot continue to type into the input unless they navigate back to it.	Focus remains on the input throughout keyboard navigation of the related list. This allows the user to refine search terms without navigating back to the input.

Checklist

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1. Assign the correct component label to this issue.
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