aconfig: Use finalized flags file for SDK check in codegen.

changes:
-change python script to rust binary, was planning on doing this at some
point anyway and it was getting too complicated to the point where it
wasn't saving me any effort
-use json (as a representation of the rust struct)  instead of proto as the intermediate isn't stored so it might as well be something simpler (kept the aconfig_internal proto in case in the future we want something stored)
-add genrule which runs the convert_finalized_flags binary to go from
many text files -> one json file
-add this genrule as a dependency on the aconfig binary, then include
the file in that binary. (note the aconfig binary runs at build time only and is
not used on-device)
-we use cargo for quick local testing, which wouldn't have access to the
genrule, so added some logic to cover this and guarded it to ensure
these methods only run when using cargo

Bug: 378936061
Test: manual, already had unit test for the codegen
Change-Id: If011d796139e28e919c80b5f531dc8febd82dd0e