Use LocalService to access UserManagerServcie from DragState.

It allows us to replace UserManagerServcie with a mock when writing
tests for DragState.

Bug: 70601660
Test: com.android.server.wm.DragDropControllerTests,
      android.server.wm.CrossAppDragAndDropTests,
      manually check the drag and drop behavior on test app.

Change-Id: I2db9cbe1e1b3542ecd7d60b26fc0e7b322041eb4