Re-add dividers when expanding notifications.

* This makes the divider consistent, given Ic181bc8b23ae62c8212fee19fd7c6434eb94cd61
* Fixes some small color/alpha changing bugs that occurred when the expansion finished and the background and dividers disappear.
* If we want to disable the dividers, we can do so quickly by setting `notification_children_container_divider_height` to 0

Bug: 191187506
Test: expand and collapse groups a bunch.
Change-Id: I773ef25ec7e0121603412aff3418843a7aa4d7aa