Remove invalid usage of DisplayList.isValid()

 Bug: 13324734

 Editor was using isValid as a mechanism to track whether or not it
 needed to re-record the DisplayList. This is not correct as isValid()
 is not a general-purpose dirty bit. Add an explicit dirty bit for
 Editor to use instead

Change-Id: I5608b151791870fca3681056b5507bd39ee48f52