synchronized should be considered a dereference

[agrieve]

(not very important bug figured it's worth an issue).

For the following code snippet:

        synchronized (sInstances) {
            manager = sInstances.get(registry);
            ...
        }

NullAway complains about the sInstances.get(), but it should complain about the synchronized (sInstances), since that's the first line that would throw a NPE if sInstances were null.