ConnectivityServiceTest: some fixes in CallbackInfo

- CallbackInfo becomes a static class so that it can be instantiated
  and looked at more easily than before.
- CallbackInfo gains hashCode() because it has equals().
- effectively final field are qualified final
- fixes bug when polling the next callback: poll() on a LinkedBlockingQueue
  returns null when timing out, which was not failing the test cleanly.

Test: ConnectivityServiceTest passes
Bug: 32561414
Change-Id: I64d382a35fe83f21bded98e6906c1ec70a8f1085