Consolidate swipe state in SwipeHelper

- Move state from AmbientState (unneeded for StackScrollerAlgorithm)
- Remove repeated state from NSSL

Bug: 171817112
Test: half-swipe, full-swipe, longpress a lot, no regressions
Change-Id: I6a4e00d69124c44c119ee10057b43185739a8ea3