Add stubs for hiddenapi

Adds stubs for ojluni classes some of whose hidden members are used by
applications. Each such member is annotated with the UnsupportedAppUsage
annotation.

The stubs are built as part of the core-oj-hiddenapi target from which
the build will extract information about the hidden class members that
are used by applications.

This was tested by making and then manually checking that the generated
out/target/common/obj/PACKAGING/hiddenapi-light-greylist.txt was the
same (after sorting) before and after this change.

Test: see above
Bug: 117818301
Change-Id: Icfe773e00ba1f96a379b705f26ef631ec34fee5e
diff --git a/openjdk_java_files.bp b/openjdk_java_files.bp
index 0b06102..e71016e 100644
--- a/openjdk_java_files.bp
+++ b/openjdk_java_files.bp
@@ -1805,6 +1805,12 @@
     ],
 }
 
+// Classes that provide information about hidden APIs.
+filegroup {
+    name: "openjdk_hiddenapi_javadoc_files",
+    srcs: ["ojluni/annotations/hiddenapi/**/*.java"],
+}
+
 // Generates stub files for the classes exposed in the public API, without
 // javadoc. This can be used as a starting point for adding a new file to
 // libcore/ojluni/annotations/sdk. See libcore/ojluni/annotations/README.