API: Enable UnhiddenSystemApi check for frameworks/base

Test: m checkapi
Bug: 115333477
Change-Id: I84d454f3495d7ab8e5f0bc2e4c24725f1cd2324f
diff --git a/Android.bp b/Android.bp
index 151adf8..2b64932 100644
--- a/Android.bp
+++ b/Android.bp
@@ -1147,6 +1147,7 @@
 metalava_framework_docs_args = "--manifest $(location core/res/AndroidManifest.xml) " +
     "--hide-package com.android.okhttp " +
     "--hide-package com.android.org.conscrypt --hide-package com.android.server " +
+    "--error UnhiddenSystemApi " +
     "--hide RequiresPermission " +
     "--hide MissingPermission --hide BroadcastBehavior " +
     "--hide HiddenSuperclass --hide DeprecationMismatch --hide UnavailableSymbol " +
@@ -1563,6 +1564,7 @@
     dex_mapping_filename: "dex-mapping.txt",
     args: metalava_framework_docs_args +
         " --hide ReferencesHidden " +
+        " --hide UnhiddenSystemApi " +
         " --show-unannotated " +
         " --show-annotation android.annotation.SystemApi " +
         " --show-annotation android.annotation.TestApi "