Carefully select which DNS servers to send to netd

Select only DNS servers that:
    - are reachable, according to routes in the LinkProperties, AND

    - have a "suitable" source address in the LinkProperites, meaning:
        - IPv4 DNS server:
            - only if LinkProperties has any IPv4 address
        - IPv6 link-local DNS server:
            - only if the server has a scopeId set
            - assume for now that LinkProperties has a suitable
              link-local address
        - IPv6 non-link-local DNS server:
            - only if LinkProperties has a global, preferred IPv6 address

Bug: 19470192
Bug: 20733156
Change-Id: Ibd95f3f7b33a4fb6c36d1cea4adb63c99068f657