Unbind from dream overlay in onUnbind.

If a dream is unbound before finish, unbinding from the overlay service
will not occur before onDestroy. This will result in an exception from
unbinding from a non registered service. This change ensures the dream
service unbinds from the overlay service when unbound.

Test: atest DreamManagerServiceTests#testForceStopStubbornDream
Fixed: 229824204
Change-Id: Id708b9119c5498bf9632b21a539c3ca7f5506737