property_contexts

wificond would like to be able to set WiFi related properties
without access to the rest of the system properties.  Today,
this only involves marking the driver as loaded or unloaded.

avc: denied { write } for name="property_service" dev="tmpfs" ino=10100
scontext=u:r:wificond:s0 tcontext=u:object_r:property_socket:s0
tclass=sock_file permissive=0

Bug: 29579539
Test: No avc denials related to system properties across
      various WiFi events.

Change-Id: I6d9f1de3fbef04cb7750cc3753634f9e02fdb71f
(cherry picked from commit 1ebfdd6a)