Pass Window to DreamOverlayService when attached.

Previously, the DreamService would send the window
attributes to the DreamOverlayService. However, it is
possible that the Window isn't fully associated with
a token at this stage.

This change ensures the Window is only passed when
the token is ready, tracked by when the DecorView
is attached.

Bug: 204605351
Test: atest DreamOverlayTest
Change-Id: Ib1d25ba55edb648739aca22d861e9522102d0668