Fix some problems with soong metrics loading

If we didn't need to run soong_build during the current run, we still
try to load the soong metrics. But in the case of `dist`, that's in a
directory that is not guaranteed to persist between runs.

Make loading the soong metrics optional if the file does not exist. Also
fixes a variable shadowing issue that meant we never passed it into
ctx.Metrics.

Test: treehugger
Change-Id: Ic836282f4d13e91daa0e7241ad7c488de3293d8b