Fix NetworkMonitor logging to include NetID.

This was broken when NetID selection was moved to after
NetworkAgentInfo construction time.  NetworkMonitor inherits
from StateMachine whose log tag is immutable after construction
making it impossible to include the NetID.  Instead define a
simple log function in NetworkMonitor that includes correct
NetID.  This makes debugging via logs much easier and avoids
ambiguities that can make logs useless.

Change-Id: Ie72fc210b92bd425184d288b35ed476669e2e073