Add SELinux policies for example service.

Add selinux policies for the brillo_example_service which is started by
init. The brillo_example_client has to be started manually as root.
Otherwise, the oneway callback from the service to the client fails.

BUG=26405163
TEST=manual test with the out of tree flow

Change-Id: I08af4f08f16dcaa36abdb441bb72d140ddf8c351