STL flingToScroll should return the consumed velocity

flingToScroll() was introduced in ag/29700350.

We initially assumed that performFling() would return the consumed
velocity, as many other Compose APIs do. However, this particular
function returns the remaining velocity.

To ensure consistency, our flingToScroll function should also return the
consumed velocity. We have updated the test to reflect this expected
 behavior.

Test: atest PriorityNestedScrollConnection
Bug: 370949877
Flag: com.android.systemui.scene_container
Change-Id: I2fe883491a0681b72030b2b64493712d24867fc8