Use `mozfun.norm.os(normalized_os)` to ensure consistency of OS normalization

`os` platforms are recorded with variance, for example ‘Windows_NT’, and Windows’ are the same, 'Darwin' vs 'Mac' (#2818). To streamline this, we should use `[mozfun.norm.os(normalized_os)](https://mozilla.github.io/bigquery-etl/mozfun/norm/#os-udf)` to ensure consistency with how we handle OS normalization.