Use RFC 7217 stable privacy addresses

Ask Netd to setup WiFi interfaces to use stable privacy addresses. This
should fail benignly on devices with kernels lacking RFC 7217 support.

Test: as follows
    - built
    - flashed
    - booted
    - Manually observed persistent stable secret
    - Manually observed randomized IPv6 addresses (with flags = 800)
Bug: 17613910

Change-Id: I9465de16685b1eb0fd842446a530cf98d77a0c28