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

Cherry-pick of Change-Id: I81926e8833c1abcb17a4d49687fc89619b416d6c

Bug: 20762975
Change-Id: I665a460f30a1ef57b513da9166aad60097dd4886

system_app.te

@@ -48,6 +48,7 @@ allow system_app anr_data_file:file create_file_perms;
 # Settings need to access app name and icon from asec
 allow system_app asec_apk_file:file r_file_perms;
 
+allow system_app servicemanager:service_manager list;
 allow system_app mediaserver_service:service_manager find;
 allow system_app nfc_service:service_manager find;
 allow system_app radio_service:service_manager find;