Log the creation time of network agents

This is useful for debugging e.g. b/219083584 where the network
agent is far older than the log start.

Example log :
  NetworkAgentInfo{network{100}  handle{432902426637}  ni{WIFI CONNECTED
  extra: } created=2022-02-26T07:30:34.393Z Score(60 ; [...]

Bug: 238139913
Test: manual
Change-Id: I65f522c24e8b4084503765d29658bab78b45166c
1 file changed