Cleaning up drag state management.

When the drag is started, the UI automatically goes into spring loaded mode. On a successful
drop, it is the responsibility of the {@link DropTarget} to exit out of the spring loaded
mode. If the drop was cancelled for some reason, the UI will automatically exit out of this mode.

Bug: 34692289
Change-Id: Ic611739a43bb8d9279b587aaee3039326c143e8b