bp2build BUILD files for build/bazel/examples/apex.

The previous build/bazel recursive glob is too aggressive. Not all
directories should be ignored by bp2build.

Test: bp2build-incremental builds //bionic/... and everything it depends on in //build/bazel/...
Bug: 193658718
Change-Id: Icb48fd687f0fe6be144caa5b8c9a4f01d15b6c20