Polish shelf tracking

- Offset distance based on where gesture started, so that the shelf is
  always under your finger even if you swiped up high on the nav bar
- Use the same distance when swiping down to launch a task as we use for
  swiping up from an app (so that the swipe feels more reversible)
- Allow swiping up in the gap between hotseat and overview, so there's
  no "dead zone"

Bug: 109709720
Change-Id: I828d82e8feb250fd31e382d42aeefcb11d4a07d5