Convert filegroups in api_bp2build

api providing modules (droidstubs, cc_library etc.) can refer to the api
file via a separate filegroup module. Therefore these modules should be
generated in the api_bp2build workspace as well

Test: m api_bp2build && build/bazel/bin/bazel build
--config=api_bp2build --config=android //build/orchestrator/apis:*

Change-Id: I77371bd94a2794770b01b98aaf84b1bc42810841