Fix issue with persistent drag window

- The drag count needs to be per-display since using a global count
  would prevent us from hiding the window for the non-last display

Bug: 169894807
Test: Drag and drop with a foldable device and ensure that no drag
      window lingers for a secondary display
Change-Id: I5bb26d07c143ab4f062deb34f1564735a095817f