Extract DNS setting logic into DnsManager class

In future, managing DNS-over-TLS hostname lookup and netd programming
can be encapsulated here.

Test: as follows
    - built
    - flashed
    - booted
    - runtest frameworks-net passes
Bug: 64133961
Change-Id: I47ccfa99c30c780524c45c4af605e720ccba34a0
5 files changed