Ignore signature stripping protection for preinstalled APKs.

The current build process may currently strip APK Signature Scheme v2
signatures from prebuilt APKs to be installed on the system or vendor
partitions. However, it leaves intact the signature scheme rollback
protections introduced by APK Signature Scheme v2. Due to a bug, when
the system extracts signer certificates from preinstalled APKs, it
encounters the rollback protection and aborts the extraction process.
This manifests itself as some preinstalled packages not appearing as
installed.

This change makes the system ignore signature scheme rollback
protections when extracting certificates from preinstalled APKs. This
is fine because the process of extracting certificates from
preinstalled APKs does not care about validity/integrity of signatures
and the APKs. It only cares about extracting signer certificates.

Bug: 27829513
Change-Id: I3bed463e776b057e93a0fce915db4014946be1f9