Split the core/platform API into stable and legacy versions.

For now, everything outside libcore still uses the legacy version.

Test: treehugger
Bug: 157640067
Change-Id: If5234e9ee533ff537926801a0af045d36b1caf01
diff --git a/java/hiddenapi_singleton.go b/java/hiddenapi_singleton.go
index 2f35798..bff591c 100644
--- a/java/hiddenapi_singleton.go
+++ b/java/hiddenapi_singleton.go
@@ -116,7 +116,7 @@
 
 	// Core Platform API stubs
 	corePlatformStubModules := []string{
-		"core.platform.api.stubs",
+		"legacy.core.platform.api.stubs",
 	}
 
 	// Allow products to define their own stubs for custom product jars that apps can use.