Add widget description to WidgetCell

Full picker: https://screenshot.googleplex.com/8Dij79K8Pq2KWdm.png
Bottom picker: https://screenshot.googleplex.com/5f9TLra49dYJjLB.png

Test: Manually launch full and bottom widget pickers and observe
      description is only shown for widgets but not shortcut.

Bug: 179797520
Change-Id: I5bfde056906a4a0f4fff9bc9a488f808365173b3