Make getExtensionVersion a little more forward-proof

R_EXTENSION_INT is 0 now, but if bumped we don't want to bump
the other return values of this method.

Bug: 154486201
Test: SdkExtensionsTest
Change-Id: I36855ba1881da4056276f4a5626ca64d2129353b

apex/sdkextensions/framework/java/android/os/ext/SdkExtensions.java

@@ -62,7 +62,11 @@ public class SdkExtensions {
         if (sdk < VERSION_CODES.R) {
             throw new IllegalArgumentException(String.valueOf(sdk) + " does not have extensions");
         }
-        return R_EXTENSION_INT;
+
+        if (sdk == VERSION_CODES.R) {
+            return R_EXTENSION_INT;
+        }
+        return 0;
     }
 
 }