Don't adjust ABI if PackageSetting#pkg is null.

If means the package hasn't been scanned yet, and we
will adjust the ABI during the scan of the last package
in the shared user group.

NOTE: This needs some more cleaning up, which will be
done along with the remaining TODO in this function.

Change-Id: Ie332806b64e22ab4a4856e1ccd064ff6a01616bf