Add missing arg to art/libcore stubs generation

These packages are meant to have everything annotated @RecentlyNullable
or @RecentlyNonNull. The "non-updatable" stubs pass this argument to
achieve that, but it was missing from the libcore stubs.

Bug: 176149340
Test: m art.module.public.api.stubs.source && \
  rm -rf /tmp/lib && \
  unzip -qd /tmp/lib *.jar && \
  javap -v /tmp/lib/java/util/concurrent/ConcurrentHashMap.class | \
  grep Recent | \
  wc -l
  (verify it is a number >100)
Change-Id: I64bfcac0f5cf19e252f2a085b45d9f4b7173c7ed
diff --git a/JavaLibrary.bp b/JavaLibrary.bp
index 0da4c80..a7e6738 100644
--- a/JavaLibrary.bp
+++ b/JavaLibrary.bp
@@ -844,6 +844,7 @@
     api_only: true,
     droiddoc_options: [
         rewrite_openjdk_doc_args,
+        "--force-convert-to-warning-nullability-annotations +*:-android.*:+android.icu.*:-dalvik.* ",
     ],
 
     // Emit nullability annotations from the source to the stub files.