Address leftover comments on 923337 and 930217.

- Restrict unprivileged apps to use
  NetworkRequest.Builder#setSignalStrength.

- Remove the "throws NullPointerException" in
  CaptivePortalProbeSpec constructor.

- Remove the null check in LinkProperties.

- Add annotataion into all ConnectivityManager.NetworkCallback
  methods.

Change-Id: Id275cac1d6a30d7515cd7b113394f5e8a0179314
Fix: 129097486
Test: atest FrameworksNetTests