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/android/api_levels.go b/android/api_levels.go
index 1fbbc15..926d297 100644
--- a/android/api_levels.go
+++ b/android/api_levels.go
@@ -344,7 +344,7 @@
 
 var apiLevelsMapKey = NewOnceKey("ApiLevelsMap")
 
-func getApiLevelsMap(config Config) map[string]int {
+func GetApiLevelsMap(config Config) map[string]int {
 	return config.Once(apiLevelsMapKey, func() interface{} {
 		apiLevelsMap := map[string]int{
 			"G":     9,
@@ -374,7 +374,7 @@
 }
 
 func (a *apiLevelsSingleton) GenerateBuildActions(ctx SingletonContext) {
-	apiLevelsMap := getApiLevelsMap(ctx.Config())
+	apiLevelsMap := GetApiLevelsMap(ctx.Config())
 	apiLevelsJson := GetApiLevelsJson(ctx)
 	createApiLevelsJson(ctx, apiLevelsJson, apiLevelsMap)
 }