This is used to persist RFC 7217 stable secrets across device reboots.

First submit caused a merge conflict. This revision replaces netd_prop
with a more unique name netd_stable_secret_prop.

Test: as follows
    - Manually tested that stable_secret is generated on first use and
      persists until reset of user data partition (factory reset).
    - Tested that "adb shell getprop" was denied access to
      persist.netd.stable_secret after running "adb unroot".
Bug: 17613910

Change-Id: I0a609c724799a15b1926e62534c16810d34f2275