Use correct cert chain from PKCS#7 SignedData block.

This fixes a bug where APK JAR signature verifier returned the wrong
certificate chain. Rather than returning the cert chain of the
verified SignerInfo, it was returning the bag of certs of the PKCS#7
SignedData block.

This issue was introduced in Android N and thus does not affect
earlier Android platform versions.

Bug: 29055836
Change-Id: I684c0f8e9ff47b922030645e07b6a114c0eb0963