Parse IFA_F_* values above 0x80.

In RTM_NEWADDR messages, the first 8 flags are reported in the
ifa_flags field in struct ifaddrmsg, but flags above 0x80 are
reported in the IFA_FLAGS attribute. NetlinkEvent currently only
looks at ifa_flags, so it cannot see higher-value flags such as
IFA_F_STABLE_PRIVACY. Fix this by parsing the IFA_FLAGS
attribute.

Bug: 155005831
Test: makes new test in aosp/1295495 pass
Change-Id: I723f1106cbcea2186fc6452305942a0f8301fd2a