Use apksigner in check_target_files_signatures

Some apk files, e.g. Chrome.apk has switched to use v2+ scheme.
And the apk file no longer has a META-INF/CERT.RSA. So, the
signature parsing script should use apksigner to dump the cert
info.

Leave the parsing of META-INF/CERT.RSA as a fallback, as some apks
fail the `apksigner verify`. The script also switches to store the
digest of the cert instead of the raw bytes.

Bug: 157735036
Test: run check_target_files_signatures on aosp|signed target-files

Change-Id: I910cd1aa91d1e446bef7097871af4171c285685d