ConnectivityService needs to allow for registering and unregistering ConnectivityDiagnosticsCallbacks as part of ConnectivityDiagnostics API. These methods will be used by ConnectivityDiagnosticsManager so that privileged applications can receive notifications on network events by the system. Bug: 146444622 Test: compiles Change-Id: Id00b30af5692db5eef47d0d948897148578d3b32