STL decay animation should only play if decayOffset exceeds targetOffset

This change also addresses sudden direction changes during fling.
The calculation to determine if the bound has been reached now only uses
targetOffset and decayOffset.

Test: atest DraggableHandlerTest
Bug: 388803469
Flag: com.android.systemui.scene_container
Change-Id: Ibaba56daee8081eb0d5ce66911f6e185aa416cc8