Enable RFC 7217 stable privacy addresses

... on kernels that support this feature. Android property server is
used in combination with SELinux policy to limit access to the
necessary stable secret.

This change also makes some small improvements and fixes to
libnetdutils.

ip -6 addr indicates stable_privacy addresses with "flags 800"

wlan0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP qlen 3000
inet6 fdd5:6241:900d:0:d547:442:33d7:1982/64 scope global temporary dynamic
inet6 fdd5:6241:900d:0:9dbe:3875:1cab:eaf2/64 scope global mngtmpaddr dynamic flags 800
inet6 2401:fa00:4:ca0:d547:442:33d7:1982/64 scope global temporary dynamic
inet6 2401:fa00:4:ca0:6281:68a2:7124:6848/64 scope global mngtmpaddr dynamic flags 800
inet6 fe80::d5a7:18ed:4ea7:50d8/64 scope link flags 800

Test: as follows
    - built
    - flashed
    - booted
    - "runtest -x .../netd_unit_test.cpp" passes
Bug: 17613910

Change-Id: I0e0b6bbd15cb9d46368bed8aef5dac2f6183d32a