property_socket: add mlstrustedobject attribute

In order to allow set_prop() to function with platform_apps,
the property_socket file requires mlstrustedobject since
platform app uses category sets.

This does not allow untrusted_app access, as the following
neverallows still prevent type access:

untrusted_app.te:118:neverallow untrusted_app property_socket:sock_file write;
untrusted_app.te:120:neverallow untrusted_app property_type:property_service set;

Lastly, the internal socket to property_service is labeled with init
which is mlstrustedsubject, so no changes are required there.

Change-Id: I47296a2dc24b16785fd296deea7a54ae9966226a
Signed-off-by: William Roberts <william.c.roberts@intel.com>