Treat optimistic addresses as global preferred.

If the kernel sends notification of an optimistic address then
treat is a useable address (isGlobalPreferred()).

Note that addresses flagged as IFA_F_OPTIMISTIC are
simultaneously flagged as IFA_F_TENTATIVE (when the tentative
state has cleared either DAD has succeeded or failed, and both
flags are cleared regardless).

Additionally: do not consider RFC 4193 ULA addresses sufficient
for "global preffered".  They are, by definition, of global scope
but not sufficient for global reachability.

Bug: 17769720
Change-Id: I759623b28fd52758f2d4d76d167f3cafd9319d6a