STL make DragController.onStop a suspended function

We now have the capability to launch onStop animations within their
respective coroutine scopes.
This improvement will allow us to manage more complex animations in the
future.

Test: Refactor DraggableHandlerTest and MultiPointerDraggableTest
Bug: 378470603
Flag: com.android.systemui.scene_container
Change-Id: I985098f2a5631b3f6d4118e318a52eca158aed8f