Switch all folder preview rendering to be programmatic (ie. no assets)

-> Refactored the preview background rendering to be much more self-contained.
   This cleans up a lot of code in the CellLayout, and keeps the logic in the
   right place.
-> We switch to software rendering for performance and compatibility reasons.
-> Removed all assets.
-> FolderIcon accept animation includes animation of the clipped region.
-> 1:1 hand-off of drawing of the FolderIcon background between the FolderIcon
   and the CellLayout. Unfortunately, CellLayout rendering is still required
   to work around clipping issues (due to use of software layer). We also
   need this to support folder creation feedback.

Change-Id: Ib8f7fa6359dfedff8145f38dd50ba03849ca0d51