Some cleanup for SwipeDetector.

It is now organized as follows:
 - private constants
 - public constants
 - private final fields
 - private variable fields
 - constructors
 - public methods
 - private methods
 - public interface/abstract class

This is intended to be a functional no-op.

Bug: 141939911
Change-Id: Iad5a9b3b73b35641f8a4f1d52ada6adef3825c47
Tested: Built and sanity checked manually.