Fix a couple polish bugs with new folder previews

-> Don't reuse the same background object for the folder create preview
   since this can cause interruptions in the animations for previous
   creation previews.
-> When drawing the background to preview creation, don't draw the stroke
   above the icon since the icon is not yet contained by the folder.

Change-Id: Ib666dc2453df465b342c02f3bd109b553a769dcc