Add AIDL for registering ConnectivityDiagnosticCallbacks.

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