Add CT log list data verification after download

The verification step assumes the metadata file, downloaded together with
the log list data, contains a signature generated using the public key
available at the certificate-transparency-community-site from Google
https://github.com/google/certificate-transparency-community-site/blob/master/docs/google/known-logs.md

Flag: com.android.net.ct.flags.certificate_transparency_service
Bug: 319829948
Test: atest NetworkSecurityUnitTests
Change-Id: If5fefb33b00c49a8f043d9b030f344f7917affbc
3 files changed