PopupSample: Moonstone Popup closes abnormally on spotlight events

Issue: _onDisappear routine is being triggered on a control which
doesn't have focus.
Analysis: _onDisappear  routine should be triggered only based on the
current focused control. If there is no _oCurrent control, there is no
need of observing any of the  _onDisappear events.
Fix: whenever _oCurrent = null , _observeDisappearance(false, _oCurrent)
is called.
Enyo-DCO-1.1-Signed-off-by: Rajyavardhan P <rajyavardhan.p@lge.com>