[#2800] Fix NPE in SingleThreadEventExecutor on Android

Motivation:

The recent changes f8bee2e94c33f167bcdefe3e8bbf95e99765032e to use a FJP introduced a regression on Android that cause a NPE.

Modifications:

- Use AtomicReferenceFieldUpdater so it works also on Android
- Fix inspection warnings

Result:

Netty works again on Android too.