Inet6Address: Provide an explicit hostname for the ANY address.

Inet6Address.ANY was being constructed with a null hostname. This
meant that the first call to getHostName() would set its hostname
to "::". This means that Inet6Address.ANY.toString() would be differ
depending on whether or not getHostName was called. While this is true
of all InetAddresses, it's especially bad behaviour for an address
object that's essentially treated as immutable.

bug: 28294969
Change-Id: I51c1e0bd3da34ac65dfc0739e969b831dc2aeb89