[avatar] Shouldn't be keyboard focusable by default

[jouni]

Description

The Avatar has tabindex=0 by default, making it a tab stop and focusable with the keyboard. This seems wrong for a purely visual element, similar to an image or an icon.

When inside an Avatar Group, it might be more expected, as the overflow item should be focusable so that the overlay can be opened with the keyboard.

[web-padawan]

Note: once we implement this change, the following overrides can be removed from vaadin-message:

web-components/packages/message-list/src/vaadin-message-mixin.js

Lines 98 to 99 in 51134af

	avatar.setAttribute('tabindex', '-1');
	avatar.setAttribute('aria-hidden', 'true');