Add getpgid to system_service and init

In libprocessgroup, we want to only send signals once to processes,
particularly for SIGTERM.  We must send the signal both to all
processes within a POSIX process group and a cgroup.  To ensure that
we do not duplicate the signals being sent, we check the processes in
the cgroup to see if they're in the POSIX process groups that we're
killing.  If they are, we skip sending a second signal.  This requires
getpgid permissions, hence this SELinux change.

avc: denied { getpgid } for pid=797 comm="ActivityManager" scontext=u:r:system_server:s0 tcontext=u:r:untrusted_app_25:s0:c512,c768 tclass=process permissive=1
avc: denied { getpgid } for pid=797 comm="ActivityManager" scontext=u:r:system_server:s0 tcontext=u:r:untrusted_app_25:s0:c512,c768 tclass=process permissive=1
avc: denied { getpgid } for pid=797 comm="ActivityManager" scontext=u:r:system_server:s0 tcontext=u:r:system_app:s0 tclass=process permissive=1
avc: denied { getpgid } for pid=797 comm="ActivityManager" scontext=u:r:system_server:s0 tcontext=u:r:system_app:s0 tclass=process permissive=1
avc: denied { getpgid } for pid=1 comm="init" scontext=u:r:init:s0 tcontext=u:r:zygote:s0 tclass=process permissive=1
avc: denied { getpgid } for pid=1 comm="init" scontext=u:r:init:s0 tcontext=u:r:zygote:s0 tclass=process permissive=1
avc: denied { getpgid } for pid=1 comm="init" scontext=u:r:init:s0 tcontext=u:r:system_server:s0 tclass=process permissive=1
avc: denied { getpgid } for pid=1 comm="init" scontext=u:r:init:s0 tcontext=u:r:system_server:s0 tclass=process permissive=1

Bug: 37853905
Bug: 62418791
Test: Boot, kill zygote, reboot
Change-Id: Ib6c265dbaac8833c47145ae28fb6594ca8545570
(cherry picked from commit c59eb4d8)