allow APKs with same packageName/versionCode but different signer

allow APKs with same packageName/versionCode but different signer

There are many APKs out in the wild that claim to be the same app and
version and each other, but they are signed by different keys.  fdroid
should be able to index these, and work with them.   This supports having
the developer's signature via reproducible builds, random collections of
APKs like repomaker, etc.