Allow shell domain to use system_server_service.

Shell domain needs to be able to access system_server_services, e.g.
when running the pm command. Addresses the following denials:

10-07 00:59:26.901   178   178 E SELinux : avc:  denied  { find } for service=user scontext=u:r:shell:s0 tcontext=u:object_r:system_server_service:s0 tclass=service_manager
10-07 00:59:26.903   178   178 E SELinux : avc:  denied  { find } for service=package scontext=u:r:shell:s0 tcontext=u:object_r:system_server_service:s0 tclass=service_manager

Change-Id: I4cc2f31809a2615ba781e2ecfe2ca7d6f5226b73