In the build signing environment, we don't necessary have access to the
physical private key. And we work around it by passing an extra
signing_helper to the avbtool instead of a private key. Since we need
to perform signing to add the hashtree footer when rebuilding the apex
image. We should propogate the signing_helper to apexer and eventually
to avbtool.
Bug: 148627666
Test: unit tests pass. Smoke test by pass a public key to apexer and
hardcode the private key in signing helper script.
Change-Id: I56d57485493a5ba2cbe4c49e493ee9604f68088b
cea6ad1fb0