Fix shadow problems with Folder animation.

The shadow from the top of the Folder is visible through
the transparent part of the 'preview' background. When we
remove the elevation as part of the animation, the shadow
jumps into visibility when the animation is done.

To solve this, we remove the elevation during the Folder animation and
 * Animate the elevation at the end of the Folder opening animation.
 * Animate the shadow of the FolderIcon bg in once the Folder is closed.

Bug: 62787582
Bug: 35064148
Change-Id: Id5d8fcbfa4f74882531334f12488560da2496faf