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

Merged-In: I920cbc7b63790c4937acc56d1811fc04026e11fd
(cherry picked from commit 6f61c9c5)

Change-Id: I3e46bf924067023920387a0e577f01cf672cac89