Add a function to fingerprint flag/offset info.

Uses SipHasher implementation added to the project. Ideally the output
would remain consistent between builds (though the worst case of an
updated algorithm would be a string lookup for cross-container reads
only).

Sort the flags first so everything is always added to the hasher in the
same order - note this code is only run at build time, not runtime.

Next step is to write this fingerprint into the package.map file and
introduce a flag to guard this change.

Bug: 316357686
Test: atest aconfig.test
Change-Id: Ie5f34541d982dfa120ffb05fc0790603689d47c0