Add comment / logging in NetworkMonitor callbacks

Add a comment explaining the ordering of messages sent to the tracker
and connectivity diagnostics handlers.

Add a Slog.wtf call in case the deprecated notifyNetworkTested callback
is called.

Bug: 153500847
Test: atest ConnectivityServiceTest
Change-Id: I2dbfc9bf7b2f785ea4594851bd354e9fd0fc0bd1