Fix use of reachable DNS server logic

Move reachable DNS server computation out of ConnectivityService
and split it into LinkProperties#isReachable() and a companion
change in WifiStateMachine's makeLinkProperties().

Restore previous ConnectivityService#updateDnses() behaviour, as
the pruning is done in WifiStateMachine now.

Bug: 19470192
Bug: 20733156
Bug: 22098233
Change-Id: I810ef74d504e5dc1ca2017d435cdadd6b82171e6