1. Add post_update.sh to preserve build.gradle.
2. Add the patches/fix_infinite_recursion.diff to fix infinite
recursion.
3. Run tools/external_updater/updater.sh update antlr

(Should we upgrade to v4?)

Test: m javac-check RUN_ERROR_PRONE=true
Change-Id: Idafdb1cf6cf2310d983be612f36c5e6596a6e23f