Pass signing_args to apexer to repack apex

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