Revert "Bump Android NDK version to r21."

This reverts commit 9822b13842a6f476f023ef282d329c276d165a7c.
diff --git a/zorg/buildbot/builders/sanitizers/buildbot_android.sh b/zorg/buildbot/builders/sanitizers/buildbot_android.sh
index 59939b5..6f2676b 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 r21
+download_android_tools r16
 
 # Stage 1