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

(cherry picked from commit 4e1619f0)

Change-Id: Ica5f1609975e4b256f4e2f3dc2ac84362e762da2