Allow system_app to list all services.

The Settings app contains a SystemPropPoker class which notifies every service
on the system that a property has changed.

Address the following denial:
avc:  denied  { list } for service=NULL scontext=u:r:system_app:s0 tcontext=u:r:servicemanager:s0 tclass=service_manager

Bug: 20762975
Change-Id: I81926e8833c1abcb17a4d49687fc89619b416d6c