Collect certificates when parsing APEX packages

This is a smallest possible change to fix the breakage.

This change will introduce a boot time regression, but unfortunately
there is no easy way around it. Will try to address the regression in a
follow-up CL.

Test: atest PackageManagerTest
Test: atest android.security.cts.PackageSignatureTest#testPackageSignatures
Bug: 155396683
Change-Id: If92b8ce6236a0ef38d228d0705ae4f5345066e64