Add callback registration in ConnectivityService.

ConnectivityDiagnosticsManager will send callbacks to
ConnectivityService for registering and unregistering them with the
system. ConnectivityService needs to do the processing for persisting
(and deleting) these callbacks on the ConnectivityService Thread, so
messages are sent to the Connectivity Diagnostics  Handler, which runs
on the ConnectivityService Thread.

Bug: 146444622
Bug: 143187964
Bug: 147848028
Test: compiles
Test: atest FrameworksNetTests
Change-Id: Ia5c8f90a60c050504e8676de9564a7607a9b03bc