Stop embedding soong_build_metrics in soong_metrics

Test: Run `m --bazel-mode-dev nothing` and query the database to make sure
soong_build_metrics will only update to its standalone column
Run `bp2build` and `b build` to make sure soong_build_metrics and
soong_metrics.soong_build_metrics will be null.

Bug: 262280865
Change-Id: Ice5115c468bda350b742c5a1d03fff66ad346cd6