Support discovering services with a NetworkRequest

A common mistake when using service discovery is to mismanage networks,
expecting to find or be able to access services on networks that may not
be available yet, or may be stale/lost.

This new API allows clients to start service discovery with just a
NetworkRequest, letting NsdManager track networks and notify when
services are found/lost for each network.

Bug: 190249673
Test: atest NsdManagerTest
Change-Id: I12e799aa48fce2d1a35f708c5617386b1cc70f21