[flexiglass] Expose NestedScrollSource in PriorityNestedScrollConnection API

This allows us to define a different check for when we are scrolling from a fling; an example is an overscroll boundary past which the fling-driven motion should end and onStop should be called, whereas if the motion is drag-driven the overscroll should ignore the boundary and continue until the finger is lifted.

Bug: 332569054
Test: verified manually that canContinueScroll implementations can access NestedScrollSource
Flag: com.android.systemui.scene_container
Change-Id: I9ab4950ad8a6f7908253f10e90d55c8d1473cd42