Move the logic of (re)evaluation of Private DNS

Moves this out of ConnectivityService and into each NetworkMonitor
(where it's more self-contained).

Test: as follows
    - builds, flashes, boots
    - runtest frameworks-net passes
    - manual testing with working and non-working hostnames behaves
      somewhat (but not entirely) as expected, and not always quickly
Bug: 64133961
Bug: 72345192
Bug: 73872000
Bug: 77140445
Merged-In: I5dc90ecfe6f6f10967b7501645ad8e030cb38982
Merged-In: Ida4967d22f0781524f0f269e30e653b8ec867258
Change-Id: Ic4322af3cb49149f2d975cb31f54b2ac7927f907
(cherry picked from commit 736353a5)