Teach LinkProperties whether it's provisioned or not.

In IPv4, a link is provisioned when DHCP succeeds. In IPv6, a
there is no such signal, because addresses and DNS servers can
be notified by the kernel at different times.

Add an isProvisioned method that returns true if we believe that
enough information has configured to use a network. For IPv6,
this requires an IP address, default route, and DNS server. For
IPv4, this requires only an IPv4 address, because we support
static configuration that doesn't have a default route or DNS
server.

To do this we use the existing hasIPv4Address method, rename the
all-but unused hasIPv6Address method to hasGlobalIPv6Address
(which is what we want anyway) and add new hasIPv[46]DefaultRoute
and hasIPv[46]DnsServer methods.

Bug: 9180552
Change-Id: Ib2f5ff8af920f7b6f1edf0e2afaaa0edce9bc72d