APK Signature Scheme v2 APK verifier.

This adds the ApkVerifier class which verifies APKs using APK
Signature Scheme v2 only. In a follow-up commit this class will be
extended to verify APKs using JAR signature scheme when necessary.

The APK verifier is designed to not just verify an APK, but also
report errors, warnings, and information about signers in a structured
way, to enable tools to surface this information to users in various
ways.

Bug: 27461702
Change-Id: I10c6ba436021d86b6dbf6d3cf44494652adacb66