Define ConnectivityDiagnosticsCallback IBinder impl.

An IBinder implementation of the ConnectivityDiagnosticsCallback AIDL is
needed so that apps can receive notfications with their registered
callbacks for callbacks coming from ConnectivityService.

Bug: 143187964
Bug: 147848028
Test: compiles
Test: atest FrameworksNetTests
Change-Id: Ie62678f794d81e1edee68977ec684a911b5070b7