Remove VelocityTracker abstraction

For older hardware with noisy touchscreens, a custom version of velocity
tracker, NoisyVelocityTracker, was needed. Since the hardware keeps
improving, and with recent bugfixes in VelocityTracker, these additional
layers of abstractions are no longer needed.

Some more improvements:
- mVelocityTracker is now never null
- VelocityTracker already clears its state when receiving ACTION_DOWN,
so no need for additional object destruction / creation. Replace these
calls with VelocityTracker.clear() instead.

Test: manual interaction with notification panel (pulling down, flinging
at various points).
Bug: 113070322

Change-Id: I186a6b1c3fc91ec6c18bc0248d6e36f76c65e564