Add rules for system_update service.

system_update service manages system update information: system updater
(priv_app) publishes the pending system update info through the service,
while other apps can read the info accordingly (design doc in
go/pi-ota-platform-api).

This CL adds the service type, and grants priv_app to access the service.

Bug: 67437079
Test: Build and flash marlin image. The system_update service works.
Change-Id: I7a3eaee3ecd3e2e16b410413e917ec603566b375
diff --git a/private/service_contexts b/private/service_contexts
index 6f33c89..c1ea51a 100644
--- a/private/service_contexts
+++ b/private/service_contexts
@@ -151,6 +151,7 @@
 storaged                                  u:object_r:storaged_service:s0
 storagestats                              u:object_r:storagestats_service:s0
 SurfaceFlinger                            u:object_r:surfaceflinger_service:s0
+system_update                             u:object_r:system_update_service:s0
 task                                      u:object_r:task_service:s0
 telecom                                   u:object_r:telecom_service:s0
 telephony.registry                        u:object_r:registry_service:s0