The "cannot resolve symbol R" problem can happen when you build the project on the command line, then switch to Android Studio. To resolve it, try Build > Rebuild Project. If you're still seeing the same error after rebuilding, try File > Invalidate Caches/Restart.
akwizgranchanged title from After build (described in issue #1515) to Cannot resolve symbol R after build
changed title from After build (described in issue #1515) to Cannot resolve symbol R after build
Sorry, I don't know what's causing this issue. I'd recommend cloning and building another Android project (pick one at random from GitHub) to see whether the issue is specific to Briar or whether there's a problem with your build setup.