genlist: fixed wrong item focus set on item_focus_set API call.

genlist: fixed wrong item focus set on item_focus_set API call.

Reset the last_focused_item on _item_focus_set_hook.
This fixes the issue which sets the focus to the wrong item when the
focus is set by an API.

Reproduction step:
elementary_test -> genlist focus -> click an item (not the 2nd item) -> click "Focus 2nd
Item after 1.5 seconds" button.
Focus is not moved to the 2nd item.

@fix