hiddenapi-mappings: Hide ReferencesHidden errors

The hidden API mapping is generated from sources that are not
API and the ReferencesHidden rule is therefore not needed.

Bug: 119556446
Test: mm hiddenapi-mappings
Change-Id: Ifc9efdaadeb0e61d2643230a36df930b21c0ecf4
diff --git a/Android.bp b/Android.bp
index 8023fc5..fe9b1d8 100644
--- a/Android.bp
+++ b/Android.bp
@@ -1562,6 +1562,7 @@
     ],
     dex_mapping_filename: "dex-mapping.txt",
     args: metalava_framework_docs_args +
+        " --hide ReferencesHidden " +
         " --show-unannotated " +
         " --show-annotation android.annotation.SystemApi " +
         " --show-annotation android.annotation.TestApi "