Refactor answer callback for async DNS query JAVA API

1. refactor AnswerCallback with a generic type
2. support onError in AnswerCallback
3. Fix minor problem reported from API Review

Bug: 124882626
Test: built, flashed, booted
      atest DnsResolverTest DnsPacketTest

Change-Id: I685c9989f8401acb63d2e83f552b2d5b20c41af0