Remove some warnings from the libcore-docs build

Ignore the set of non-standard JavaDoc tags from OpenJDK in the
libcore-docs build. They are non-fatal, but distracting.

Bug: 32128132
Test: make libcore-docs
Change-Id: I4923b5b4bd1063baeccef4814e8e817d03a4ba06

JavaLibrary.mk

@@ -440,6 +440,7 @@ LOCAL_DROIDDOC_OPTIONS := \
  -title "libcore" \
  -proofread $(OUT_DOCS)/$(LOCAL_MODULE)-proofread.txt \
  -todo ../$(LOCAL_MODULE)-docs-todo.html \
+ -knowntags ./libcore/known_oj_tags.txt \
  -hdf android.whichdoc offline
 
 LOCAL_DROIDDOC_CUSTOM_TEMPLATE_DIR:=build/tools/droiddoc/templates-sdk