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

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: I4dad00fb189d697aceaffae49ad63987c7e45054