Make sure callback is only called once

Because we set it already in the future, it was called already
when we fetched the contents of the future, because it looked
like we overrided the callback. Instead, don't do callbacks
when calling the future, and save the callback in a separate
field.

Bug: 25568693
Change-Id: I8b7fd4248a2336470f8b7e12b64b0577dee96f6f