Fix new Physical UX feature flagging

- Re-use the scroll-feedbacak api flag for widget scroll haptics: that
  way, it helps us to centralize the flagging/rollout of scroll haptic
  feedback.
- Create and use a new feature flag for the unflagged differential
  motion fling feature.

Bug: 293332089
Bug: 287914819
Test: atest DifferentialMotionFlingHelperTest
Test: manual flag flip to check features enabled/disabled as expected
Change-Id: I1da7052727c9fcebab2d28b66a65fb6727a16f48