1. add test case for testing bypass Private DNS
2. add null network test
3. minor change for cleanup
Bug: 130594022
Test: atest DnsResolverTest
Merged-In: I8dd48f11baf92d953ded237204a3c2cd3b58581d
(cherry picked from commit da665a0a78a2919c3e2edabafec463de3de9ddb7)
Change-Id: Iabb100c51fd80eca5ab0284bc891f4fb98492e8b