Improve overscroll performance

This CL improves the performance of STL when overscrolling: before this
CL, releasing the finger during an overscroll would make the last
animation frame (at progress 0% or 100%) slower because the
overscrollSpec would go from a non-null value to a null value, only for
that frame, placing more elements than during the transition.

This CL changes the check done to compute SwipeTransition.isBouncing()
so that overscrollSpec does not unnecessarily change values at the end
of the transition.

See b/345632645#comment5 for a before/after comparison.

Bug: 345632645
Test: DraggableHandlerTest
Flag: com.android.systemui.scene_container
Change-Id: I0b16b432a610f5613d448d00f085c723926929ee