fix(ui5-avatar): fix click event fired twice

[dobrinyonkov]

We used to fire a custom event, but did not stop the native one and end up firing two "click" events.
Now, the native one is stopped properly (we need to fire the custom one, so the noConflict ui5-click is also fired).

Also additional fixes have been performed:

• the content no longer scrolls when the SPACE key is pressed over "interactive" avatar.
• AvatarGroup now checks the target of the click event in order to detect whether an avatar or the overflow button is clicked.

FIXES: #2943