Make getdnsnetid returning app_netId instead of dns_netId and fix nits

1. getdnsnetid command return app_netId instead of dns_netId.
2. fix nits for ag/7691940
3. Add more tests

Bug: 129530368
Test: built, flashed, booted
          system/netd/tests/runtests.sh

Change-Id: Ifd1345e1124088179e38346e3693957a8b5ef63b