do not trigger mask from invalid drags

--DragDropArea in containment should not activate
mask update (animationsNeedLength) if the dragging
is not valid for that DragDropArea. A good example
for this is when the user drags Tasks, in such case
the DragEnter signal is ignored and the DragLeave
may not reach to containment. In such case the
animationsNeedLength signal should NOT be sent

BUG: 408926