Fixed the wrong scroll signal sequence in ItemView

Fixed the wrong scroll signal sequence in ItemView

[problem]     The scroll signal sequence is wrong in ItemView. while scrolling ItemView
              by panning, ScrollCompleted signal is emitted before ScrollStarted signal.
[cause]       The touch down event always causes the ScrollCompleted signal to be emitted
              because it assumes the touch down event is to cancel the ongoing scrolling.
              But actually you need to touch down first in order to start the pan.
[solution]    Only emit the ScrollCompleted signal when there is an ongoing scroll animation.

Change-Id: Ib641698b1b3771cfce53d54cec42423fb228ce17
Signed-off-by: Adeel Kazmi <adeel.kazmi@samsung.com>