Bump Android NDK version to r21.
diff --git a/zorg/buildbot/builders/sanitizers/buildbot_android.sh b/zorg/buildbot/builders/sanitizers/buildbot_android.sh
index 6f2676b..59939b5 100755
--- a/zorg/buildbot/builders/sanitizers/buildbot_android.sh
+++ b/zorg/buildbot/builders/sanitizers/buildbot_android.sh
@@ -25,7 +25,7 @@
 
 clobber
 
-download_android_tools r16
+download_android_tools r21
 
 # Stage 1