build: check AllowedAPKSigningKeys in reproducible build scenario

The builder should check the `AllowedAPKSigningKeys` at build time, so
that the CI can check if somebody gives a wrong value that doesn't match
a compared RB binary.  In the event it fails, it gives useful
information, and in the event it succeeds, it makes it clear that this
build has verification back to the developer's original key.

Also, add tests for this to the test suite.
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