Create api_domain module type and its bp2build converter

This module does not have any build actions in Soong yet. It will be
converted to a Bazel target using bp2build, and API export will be run
on the generated Bazel target

Test: go test ./bp2build
Test: TH
Change-Id: I76df32bd9f23b60e746b2700af07668a2ed74844