Expose findTrustAnchorBySubjectAndPublicKey

This allows for faster lookups of TrustAnchors when checking pin
overrides without needing to iterate over all certificates.

Currently only the system and user trusted certificate store are
optimized to avoid reading the entire source before doing the trust
anchor lookup, improvements to the resource source will come in a later
commit.

This also refactors System/UserCertificateSource to avoid code
duplication.

Change-Id: Ice00c5e047140f3d102306937556b761faaf0d0e