Implement stubs.symbol_file and stubs.versions for cc_library_shared bp2build.

This CL turns the stubs.symbol_file and stubs.versions properties into
stubs_symbol_file and stubs_version attributes on the cc_shared_library
target. See associated build/bazel change on how these attributes are
used to generate stub libraries.

Bug: 207812332

Test: New tests
Test: CI
Change-Id: Ie23eafb9903a131d92ff4e251215e998cea0a763
diff --git a/bp2build/conversion.go b/bp2build/conversion.go
index 81a4b26..96c12d3 100644
--- a/bp2build/conversion.go
+++ b/bp2build/conversion.go
@@ -1,6 +1,7 @@
 package bp2build
 
 import (
+	"encoding/json"
 	"fmt"
 	"reflect"
 	"strings"
@@ -27,6 +28,13 @@
 
 	files = append(files, newFile("product_config", "soong_config_variables.bzl", cfg.Bp2buildSoongConfigDefinitions.String()))
 
+	apiLevelsContent, err := json.Marshal(android.GetApiLevelsMap(cfg))
+	if err != nil {
+		panic(err)
+	}
+	files = append(files, newFile("api_levels", GeneratedBuildFileName, `exports_files(["api_levels.json"])`))
+	files = append(files, newFile("api_levels", "api_levels.json", string(apiLevelsContent)))
+
 	return files
 }