Copy soong_build metrics into build.trace.gz

Make soong_ui read soong_build_metrics.bp to extract the event timings
and propagate them to Tracer, which will put them in build.trace.gz.
This provides much better visibility into what parts of the build are
contributing to the overly large analysis time.

Test: m nothing and examine build.trace.gz
Change-Id: I473727f1ec044b0d973f2cb4e3eaca96bfca94f6