Extract compute(Swipes,SwipesResults) in SceneGestureHandler

This CL extracts computeSwipes and computeSwipesResults from
SceneGestureHandler. This is a pure refactoring, and those functions
will be used in a follow-up CL to change the interruption logic and
sometimes disable it (see b/322291138 for details).

Bug: 322291138
Test: PlatformComposeSceneTransitionLayoutTests
Flag: N/A
Change-Id: Id84126448dd2fb83e664daaa3b330c885d4862d8