Pinch to zoom out into overview mode.

There are 3 classes coordinating the gesture: PinchToOverviewListener,
PinchThresholdManager, and PinchAnimationManager.
- PTOL listens for the pinch gesture on DragLayer.
- When a pinch is detected, the PTOL keeps track of the interpolated
  progress and passes it along to both the PTM and PAM.
- The PTM uses the progress to determine whether a new threshold has
  been passed, and tells the PAM to animate it if so.
- The PAM uses the progress to animate things like workspace scale
  throughout the pinch.
- If the pinch ends early, the PTOL uses the last passed threshold to
  determine whether to animate to workspace or overview, and tells
  PAM to perform the animation at the same velocity as the pinch.

Bug: 24414635
Change-Id: I9e53706c705f8b2982409bf7c223d8d0e9618bf0