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