Bring the task forward when it consumes a cross-window global drag

- Send a notification when a successful cross-window drag occurs over a
  task so that we can bring it forward

Bug: 320797628
Test: atest DragDropTest DragDropControllerTests CrossAppDragAndDropTests
Test: atest WMShellUnitTests
Change-Id: Id2d2258a1e71b6dbc9a57e2f58ef43825cfc18c7