Expose the doctags that apply to the ART public API stubs

Adds a doctag_files property that lists files containing information
about the Javadoc tags that are used in the stubs.

Bug: 168301990
Test: Built offline-sdk-docs with and without the change and
      diffed them. The only difference was the timestamp.js
      file.
Change-Id: I3bb7b34081768c5387aacd168b29242c978b418c
diff --git a/JavaLibrary.bp b/JavaLibrary.bp
index 3d36425..57f27d7 100644
--- a/JavaLibrary.bp
+++ b/JavaLibrary.bp
@@ -847,6 +847,10 @@
         "ojluni-annotated-sdk-stubs",
     ],
 
+    doctag_files: [
+        ":known-oj-tags",
+    ],
+
     errorprone: {
         javacflags: [
             "-Xep:MissingOverride:OFF",