Only parse an APK once

During package installation, we were parsing the APK twice; once
in the context of the PackageInstaller and once in the context
of the PackageManager. Instead, the installer should just pass
the certificates to be used further in the process.

If the PackageManager doesn't receive certificates [or, if there's
an error using them], it will fallback to re-parsing the APK.

Bug: 27502465
Change-Id: I94ce551af54eaa9916228e933134debe50867d21