Changes COMPILE_SDK to S.

Change-Id: I5d4d84027904a7d7861e897ace1f7ce9fc04f4a4
diff --git a/gradle.properties b/gradle.properties
index 7a51375..7f4c609 100644
--- a/gradle.properties
+++ b/gradle.properties
@@ -10,4 +10,4 @@
 PROTOBUF_DEPENDENCY=com.google.protobuf.nano:protobuf-javanano:3.0.0-alpha-7
 
 BUILD_TOOLS_VERSION=28.0.3
-COMPILE_SDK=android-R
+COMPILE_SDK=android-S