Use the same key between the tile and its spacer when dragged

I previously used a different key to stop the spacer from animating, but a recent update to the DragAndDrop api revealed a bug with this approach where the tile would appear blank when dropped. Using the same key fixes this issue.

Flag: com.android.systemui.qs_ui_refactor_compose_fragment
Fixes: 390155330
Test: manually dragging tiles
Change-Id: Id05e1836147ae18484042fc9671411763ad0ccd1