Unbind from overlay before checking attached.

Currently, DreamService#finish exits early if the window is not
attached. The unbind from the dream overlay service follows this
condition. As a result, it is possible that the service is not
unbound if the window was never attached. This changelist addresses
This potential connection leak by having the unbind precede the
check.

Fixed: 227498355
Test: manual
Change-Id: I7a73258cea5219bea96f4de3b90b4b5a66478fde