releasetools: check_target_files_signatures.py checks APEXes.

Only the container certs will be checked and reported. For the payload
within an APEX, we can't easily extract the cert info.

It needs to go along a longer path, if ever needed, by:
 - extracting public keys from all the available certs;
 - using each of them to verify against an APEX payload to find a match
   (`avbtool verify_image --image payload --key public_key`).

Bug: 123716522
Test: Run check_target_files_signatures.py on target_files with APEXes.
Change-Id: I2ef318e05433d2d65ab84e2dff9e01fb6ee3373d