Add ScrollController in PriorityNestedScrollConnection

Now, `onStart` needs to provides a `ScrollController` to manage the
scrolling.

A PriorityNestedScrollConnection ensures only one ScrollController is
active at a time, guaranteeing methods are called in the correct
sequence.

The ScrollController provides control over the scroll gesture.
It allows you to:
- Scroll the content by a given pixel amount.
- Stop the scrolling with a given initial velocity.
- Cancel the current scroll operation.

Test: Refactor. Manually tested on the demo app and Flexiglass
Bug: 371984715
Flag: com.android.systemui.scene_container
Change-Id: I86724b73f4178995c39d167daea0a7de6370f7d4