Make OutDir() and SoongOutDir() be consistent.

They both used to return out/soong. Unfortunately, the tests seem to
assume the old behavior, so I had to keep tests inconsistent and I was
scared of dexpreopt, too.

Test: Presubmits.
Change-Id: Ib00c7956fd2cdb28cc11d481c17e5a3ff5ad563a