Fix service resolve on tethering downstreams

Tethering downstreams do not have NetworkAgents, and although they have
a netid of 99, Networks with netId 99 are not usable by apps for most
connectivity APIs.

Recent refactoring in NsdService adds the Network of a found service
into its NsdServiceInfo, and uses that network to resolve the service.
In that case the Network has netId 99 and resolving the service fails.

Avoid that problem by:
 - Keeping the Network field null when a service is found on a tethering
   downstream; this avoids giving apps a confusing and unusable Network
   with netId 99
 - Using the interface index found during discovery to resolve the
   service, if the app uses the NsdServiceInfo that was obtained from
   discovery to resolve. If not, all interfaces will be used to resolve,
   as per legacy APIs.

Bug: 233979892
Test: atest NsdServiceTest
      Also manual test with 2 devices connected via hotspot
Change-Id: Idd176153b67ccbd1d4f1b1fd66dafaa2f3a9e27a
(cherry picked from commit 1a8ee102)
Merged-In: Idd176153b67ccbd1d4f1b1fd66dafaa2f3a9e27a