Remove the dependency on a number of env vars.

This is so that the way soong_build is invoked is simpler, which is in
turn useful so that it can be invoked multiple times within a single
build, which in turn will be used to invoke bp2build routinely (as
opposed to manually when needed)

Test: Presubmits.
Change-Id: Iddaebb05ff7bcedc0db8273192bb31284b739920