(cid#1326323 Unguarded read)
Just switch this to an AtomicInteger, it's cheaper, and doesn't require synchronization,
so less chance of a deadlock.

This is an API change since this is a protected field in a public class, but anyone
messing with the internals of this class should have known better.

Change-Id: Idafc760c2e9d83442b8209ad23d180acb8dccb20