Clean up DIST_DIR usages, move to dist-for-goals

soong_ui now guarantees DIST_DIR is set in the environment, so remove
the default.

Use dist-for-goals instead of writing directly into DIST_DIR.

When building the system image, use DIST_DIR from the environment
instead of from make. This will eventually stop working, but this will
work for now.

Bug: 117463001
Test: m dist  (check gpl_source.tgz, logs/product_copy_files_ignored.txt)
Test: m out/target/product/generic/product_copy_files_ignored.txt
Change-Id: Ice557e0a148602cb8eb154efb747d416f0d0db59