Label /data/misc/wifi/sockets with wpa_socket.

This will ensure that any sockets created in this directory
will default to wpa_socket unless a type_transition is defined.
Define a type transition for system_server to keep its separate
system_wpa_socket type assigned for its socket.  Allow wpa
to create and unlink sockets in the directory.  We leave the
already existing rules for wifi_data_file in place for compatibility
with existing devices that have wifi_data_file on /data/misc/wifi/sockets.

Change-Id: I9e35cc93abf89ce3594860aa3193f84a3b42ea6e
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>