Since Lollipop, routes are isolated within Networks. Flushing a
Network's DNS cache whenever that same Network's routes are updated
doesn't provide any benefit. Any system components depending on this
behaviour need to uncovered and fixed.
Additionally, clean up no-longer-used flushNetworkDnsCache(). This
should be replaced, when needed, by a proper binder interface to netd.
Change-Id: I34bf79e4839da014d466058a876d754209d0c007
3d4ed1573a