Allow exporting bazel mixed build allowlists to simple text files

So that they the allowlists can be read from build_mainline_modules_bundles.sh.

Bug: 262302253
Test: m out/soong/bazel_prod_mixed_builds_enabled_list.txt out/soong/bazel_staging_mixed_builds_enabled_list.txt && cat out/soong/bazel_prod_mixed_builds_enabled_list.txt && echo && cat out/soong/bazel_staging_mixed_builds_enabled_list.txt
Change-Id: Id5f89ebb651a52dc5368681a20d6183703e05bd4