Gallery2: Fix undo button enable condition

When jump into interface of Image edit, the undo button is enable
at first, for the judgement of canUndo is set true, but actually
there is nothing to undo.

Change canUndo's judging condition.

CRs-Fixed: 759431

Change-Id: Idae9feaca62b5cdc67610cf332c2b65f1692dbb4