Generate file to map codenames to API levels.

The NDK stub library generator needs to know what unreleased API
levels exist and what the ordering is between them. This singleton
will later be expanded to generate api-level.h to cut down on
duplication.

Test: make out/soong/api_levels.json && cat out/soong/api_levels.json
Bug: None
Change-Id: I53126dd2cacb67c331c44f7d7c77c98b176b93cd
diff --git a/android/config.go b/android/config.go
index 686eeb8..3c25485 100644
--- a/android/config.go
+++ b/android/config.go
@@ -353,6 +353,10 @@
 	return strconv.Itoa(c.PlatformSdkVersionInt())
 }
 
+func (c *config) PlatformVersionAllCodenames() []string {
+	return c.ProductVariables.Platform_version_all_codenames
+}
+
 func (c *config) BuildNumber() string {
 	return "000000"
 }