Improved handle touch interaction.

- the handle size is now entirely independent of asset size
- the handle doesn't have to have a handle graphic
- the handle top/bottom sizing can each change based on
  whether the challenge is visible
- now, the handle becomes very small when the challenge is
  visible, and does not intrude into the challenge rect
  itself (so you really need to drag downward through the
  top edge of the challenge to activate the handle and close
  the challenge)
- when the challenge is closed, the handle may be clicked to
  open

Bug: 7428215
Change-Id: I4ca7b4d5d475337295a6a7492a831abbba288ac5
Proto-Id: Ibc8997ece27ca1e0fa4deb651bb11860f95d7b23