Generate a BUILD file for every Android.bp file in api_bp2build

workspace.

This is necessary to solve bazel package boundary issues where the api
file might exist in a different directory

Test: m api_bp2build && build/bazel/bin/bazel build --config=android
--config=api_bp2build //build/orchestrator/apis:system
Test: multitree_build system/nothing (in multitree)

Change-Id: Id64085d65a1943bdb394ea80c875db96ca373839