Add network created callback support

Create a network callback to notify network agent after
netd has created the network, so that the NetworkAgent may
wait for this callback and ensure the pre-work to communicate
with netd completed.

Bug: 178725261
Test: make update-api
Change-Id: I8c2ff7bd6980ae838abc5669f9419d62741b8666
CTS-Coverage-Bug: 178725261