DreamService API revisions.

Reduce reliance on Service interface overrides, instead
steering clients to the DreamService-specific lifecycle
hooks:

  onAttachedToWindow .. onDreamingStarted ..
  onDreamingStopped .. onDetachedFromWindow

The old Dream.java is finally gone now too.

Bug: 7281802
Change-Id: Ib7802c3397fde60ad1132fa49831da182eef4d7a