Support discover/resolve on specific networks

This adds a Network member to NsdServiceInfo, allowing discovered
services to report which network they were discovered on, and clients to
specify which network to resolve the service on. If clients use the
discovered NsdServiceInfo to resolve a service, it will be resolved on
the network where it was discovered instead of an unspecified network.

Also add a network parameter to a new overload of
NsdManager#discoverServices, so that clients can discover on specific
networks.

Bug: 190249673
Test: atest NsdManagerTest

Change-Id: Idc4bf9fde0f4b0328204a8cd2eedc12fffbbbdba