platform/frameworks/base/ - S is now 31

Bug: 171506470
Test: Build
Change-Id: I46d4df5f27783a96a58c966b113391ea3bc112cb
diff --git a/tools/aapt/SdkConstants.h b/tools/aapt/SdkConstants.h
index 04fbbe1..955581c 100644
--- a/tools/aapt/SdkConstants.h
+++ b/tools/aapt/SdkConstants.h
@@ -46,6 +46,7 @@
     SDK_P = 28,
     SDK_Q = 29,
     SDK_R = 30,
+    SDK_S = 31,
 };
 
 #endif // H_AAPT_SDK_CONSTANTS