Improve animating from recents to multi window state.

Previously all windows would start animating from a single thumbnail
that was clicked. Now each animates from its thumbnail, including
properly animating the thumbnail header. This involves System UI
providing information about the thumbnail setup and app transition code
using it to setup each individual animation.

Change-Id: I6f45c94af6bcbf6898b8ed757d83505af99ed6b1