Fail early if we don't have any rules file on disk.

This saves the time for parsing the APK file when there are no rules. This
probably doesn't matter much in normal use cases but in test environments (e.g.,
see bug linked), where package verifier is disabled, this avoids the
performance (install time) regression introduced by integrity check.

BUG:146874731
Test: manual comparison of install latency through inspecting logcat.
Change-Id: I3d687486f34214ac61058908401649939560b4fb