Add a post-build step for dist builds that records what changed in the build.

This will be used to skip tests when related files (or nothing) has changed.

Also minor refactoring of evaluateWhatToRun to put the logic all in one place.

Change-Id: Ia56b251716f6b6640bc5526a597cf7c066231e99
Test: go test, m (with soong tests), manually verify