Fix assert after giving focus to a disabled item.

If an item has focus stolen by another item remove activeFocus from it even
if the item that gains focus doesn't gain activeFocus.  Otherwise the focus
tree will enter a state where an item that is not the subFocusItem of its
focus scope has activeFocus which is invalid and will trigger an assert
in QQuickWindowPrivate::clearFocusInScope().

Task-number: QTBUG-34779

Change-Id: I72408ec0e4fd9b05ef595147ef1ef95b6aed1c16
Reviewed-by: Alan Alpert <aalpert@blackberry.com>