Refactor the creation of soong_build calls.

They were a bit repetitive.

In addition, make the environment checks more correct; they could
probably use an integration test or two to make sure that when the
environment changes, exactly those outputs are rebuilt that need to be,
but for now, this is an improvement already.

Test: Presubmits.
Change-Id: Idd79b81ca6975d57d00e5bf4699d266152505ff8