Children should have backgrounds

To add a background to children of a group, a couple of things
are changed when a group is in the expanded state:
- The parent's shadow + background is removed
- The group header is given a background and elevation
- The children are elevated to cast a shadow and have backgrounds
- When it's fully expanded the dividers won't be shown
- There's extra height added to the parent so that the child
  may cast the bottom shadow of the group
- As the children move into the bottom stack the last visible
  one alters its height and the ones below it are hidden
  to achieve the clipping effect

Fixes: 27591195
Fixes: 28655641
Change-Id: I0484308843e9b8bc10391387e54de07973a48f7d