platform/build/soong - S is now 31

Bug: 171506470
Test: Build
Change-Id: I7b02ab24d12ea07463efd319a7ff059751416990
diff --git a/android/api_levels.go b/android/api_levels.go
index 84ab27c..93583bc 100644
--- a/android/api_levels.go
+++ b/android/api_levels.go
@@ -289,6 +289,7 @@
 			"P":     28,
 			"Q":     29,
 			"R":     30,
+			"S":     31,
 		}
 
 		// TODO: Differentiate "current" and "future".
@@ -331,6 +332,7 @@
 			"P":     28,
 			"Q":     29,
 			"R":     30,
+			"S":     31,
 		}
 		for i, codename := range config.PlatformVersionActiveCodenames() {
 			apiLevelsMap[codename] = previewAPILevelBase + i