ENYO-266 Delay unchecking item until checking is done.

Issue
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Calling setSelectedIndex() directly and tapping checkboxItem have difference behavior.

Cause
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Tapping checkboxItem will select item first and then de-select previous item.
However calling setSelectedIndex() will de-select first in some specific case.

Fix
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To guarantee same behavior between tapping checkboxItem and direct calling of setSelectedIndex(),
we should prevent unchecking work before checking is finished due to Group.active updating.

Enyo-DCO-1.1-Signed-off-by: David Um david.um@lge.com