Create config_setting per apex_name

These are created by bp2build in /build/bazel/rules/apex. Eventually
these config_settings should likely be colocated with the source apex
definition.

Another alternative was to make Bazel's apex a macro that generates a
config_setting. I did not pursue this further for now since it requires the
apex_available of every allowlisted cc_library to also be allowlisted.
This might not always be true (e.g. com.android.runtime)

Test: go test ./bp2build
Change-Id: Ibbb14b0d9c1491b3c79b7634a18d9d35b03922c1