Expose fromContent and toContent to UserActionDistance (1/2)

This CL is a simple refactoring of UserActionDistance to make it expose
the fromContent and toContent of the transition for which we are
computing the distance.

Bug: 376438969
Test: atest DraggableHandlerTest
Test: atest SwipeToSceneTest
Flag: com.android.systemui.scene_container
Change-Id: If5dea326487b4ac53dfbfbbc445aecf2552f929b