Improved accessibility behavior of the notification shelf

The content description of the shelf was hiding the icons,
so we removed it again.
Also added an expand action to make it clear that you can expand it.
Also made scrolling in the normal shade better again by marking it
to hide it's children.

Test: manual, test accessibility on the lockscreen
Change-Id: If96c014609f3fa1b7eedb5e89acd8fe55d5b9c69
Fixes: 36660851