releasetools: Validate A/B OTA payload signatures.

$ PYTHONPATH=$PYTHONPATH:system/update_engine/scripts \
    ./build/make/tools/releasetools/check_ota_package_signature.py \
    build/target/product/security/testkey.x509.pem \
    out/dist/aosp_marlin-ota-eng.zip

Package: out/dist/aosp_marlin-ota-eng.zip
Certificate: build/target/product/security/testkey.x509.pem
...
Whole package signature VERIFIED

Verifying A/B OTA payload signatures...
...
Payload signatures VERIFIED

Bug: 65261072
Test: Signed a package and its payload with the right keys; ran the
      command above.
Test: Signed the payload with a different key; ran the command above and
      observed the reported verification failure.
Change-Id: If626ecb327a9826cd0956eef94914c939068a7d1