[fake_ddk] Run the unbind hook in a new thread.

To more closely match the device manager behavior, DdkAsyncRemove
should return immediately. This will be needed to migrate the
i2c-hid driver to the new Unbind hook.

Sometimes users will call device->DdkRelease() before fake_ddk->Ok(),
in which case they also need to call device->WaitUntilRemove() before
the DdkRelease().

BUG=37831

Change-Id: I9ed41b30d6ead7d195b85361cde400b48b932045
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/390621
Reviewed-by: Tess Eisenberger <teisenbe@google.com>
Testability-Review: Tess Eisenberger <teisenbe@google.com>
Commit-Queue: Jocelyn Dang <jocelyndang@google.com>