Use libcore/known_oj_tags for the javadoc stage.

Helps us process (and discard) OpenJdk specific javadoc tags.

(cherry picked from commit 2c4f2843f4aa904c27dc78086bff039dbd295959)

Change-Id: Ib90ce70509fefa24608067db18cfc21c6c5d0590
diff --git a/Android.mk b/Android.mk
index ed3f929..5e405d3 100644
--- a/Android.mk
+++ b/Android.mk
@@ -734,6 +734,7 @@
 # not be referenced in the documentation.
 framework_docs_LOCAL_DROIDDOC_OPTIONS := \
     -knowntags ./frameworks/base/docs/knowntags.txt \
+    -knowntags ./libcore/known_oj_tags.txt \
     -hidePackage com.android.org.conscrypt \
     -since $(SRC_API_DIR)/1.xml 1 \
     -since $(SRC_API_DIR)/2.xml 2 \