bp2build: make generated BUILD files publicly visible.

This is obviously not good build hygiene, but since Soong modules are in a global namespace, making BUILD files fully public will increase our velocity during the initial migration phases.

We'd want to lock visibility down to private by default as soon as possible, but for the purposes of migration, these will be public.

Test: bazel build //bionic/...
Change-Id: I99d62f3f8826712240decd325928435333870352