Dump netstats.proto to bugreport

Since netstats.proto is going to remove from incident.proto.
To keep netstats.proto information for debugging, dump netstats
proto as an independent file in bugreport.

Ignore-AOSP-First: CL in the same topic got merge conflict.
Bug: 222625935
Test: - generate bugreport and check the netstats.proto is there.
      - verify the output file can be parsed by printproto command.
      - atest CtsIncidentHostTestCases:IncidentdTest
Change-Id: I3ef024ac39f81ab086586697f4ce8b253c21192f