Panel physics changes.

- You can start pulling down another panel before the first
  one has fully expanded. (Protip: tap many times on the
  statusbar to invoke Schwarzenegger Mode.)
- When starting to pull down a panel, other panels are
  immediately sent running (rather than placing them on a
  literal seesaw).
- Rubberbanding is a little less aggressive: if it looks
  like you're moving your finger upward, we'll just close
  the panel outright rather than sticking to the bottom of
  the visible content. (tablets only)
- This has some implications for the background fade; you'll
  see a brief increase in brightness as you swap panels
  because the fade fraction is based on the sum of all the
  panels' fractional visibility. At times there will not be
  enough "panel" visible, in the aggregate, to justify
  holding the fade steady.

Bug: 7260868 // can't pull down panel, possibly fixed
Bug: 7204435 // double-swipe for QS
Bug: 7179458 // fling & rubberbanding heuristics
Bug: 7172453 // collapse other panels when dragging a new one
Bug: 7221970 // grabbing a flying panel causes twitching
Change-Id: Iad7c1f92c4edab9102cdda45605ef0ead4cc16c5