aconfig: Parse string to full sdk version

Re-implement Build#parseFullVersion for build-time use in Rust. Also
included the unit tests to ensure the behavior is the same. This logic
can be removed once we have an int -> Build.VERSION.X map available at
build time.

Next step is to use this in the Java codegen.

Test: atest convert_finalized_flags.test
Bug: 378936061
Change-Id: Ie9e79b4aa1b3047df97c12c9d25eb8287e30e973