Add ConnectivityDiagnosticsManager to System Server.

ConnectivityDiagnosticsManager should be accessed through
Context#getService. In order for this to be possible, it needs to be
defined as a service inside SystemServiceRegistry.

Bug: 146444622
Test: compiles.
Test: CTS testing in aosp/1211164
Change-Id: I6fe29441ecc9967a04ceb394b3bbe54830bef079