[FIX] web: dropdown unusable by screen readers

Dropdown menus/items were refactored and moved to OWL when migrating to
15.0. Now, the focus is always on the dropdown button, which captures
and handles all keyboard events, to be able to move over the menus.
Currently-selected item is highlighted by toggling classes.

However, since the focus is always on the dropdown button, screen
reader users are  no longer able to know what menu item is being
selected, as it's only a visual change (in previous versions, the focus
actually moved to the selected element).

The above is fixed by making the activeElement follow the visual change.

Part-of: odoo/odoo#105163
Co-authored-by: Luis González <lgonzalez@vauxoo.com>