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.

(cherry picked from commit 6609990e35b11c38f55f6e632160d4f2ff201ea3)

Change-Id: Ibace7849485865054e062d2b979f320bf89ff0f3