Unhide drag flag used for hiding the source task

- This flag is used by the privileged system assistant apps to
  temporarily hide its window when it starts a drag to enter
  multi-window

Bug: 371206207
Bug: 350016003
Flag: EXEMPT adding supports_drag_assistant_to_multiwindow in this CL
Test: atest WMShellUnitTests:DragAndDropPolicyTest.java
Test: atest DragAndDropPolicyTest
Change-Id: I8c5ce30745a2ff6270df5ea32c6cfdd45f8a01e9