Returns signing version from ApkSignatureVerifier

This is a first step at a larger goal of moving instant app
verifications from parsing logic into install logic.

Test: manual - install v1 and v2 instant app and static lib
Test: android.appsecurity.cts.PkgInstallSignatureVerificationTest passes.
Change-Id: Iab50b91a6fb8ef014b573bb9f733d30c1aa6022f
Bug: 68860689