Clear DreamOverlayCallback reference on destroy.

This changelist ensures DreamService doesn't reference
DreamOverlayCallback after it's destroyed, allowing it to be garbage
collected before the DreamService.

Test: manual - ensured dream and overly properly entered and exited
across invocations and selections.
Fixed: b/291723667

Change-Id: I54309e539f4417655e0d1dd47c5190494b5c6715