Add SigningInfo class to expose package signing details.

Attempt to simplify the exposure of package signing information via
PackageInfo by creating a new class and corresponding methods for
querying a package's signing information.

Bug: 74831530
Test: PkgInstallSignatureVerificationTest
Change-Id: Idbc008b41a921f89cefb224b26f910da4d238dea
14 files changed