Always allow System user to change device config in case of Multi-user-multi-display (MUMD)

When visible background user (aka MUMD) is enabled, background users are visible on other displays. In this config, some namespaces (right now game_overlay) are not allowed to be updated by any user other than current user. The CL allows SYSTEM user to change that namespace config.

Flag: EXEMPT bug fix

Bug: 375232833
Bug: 361502556
Bug: 328686115
Test: atest GameManagerStatsTests
Test: atest AndroidCarApiTest:android.car.apitest.DeviceConfigTest
Test: atest --user-type secondary_user_on_secondary_display AndroidCarApiTest:android.car.apitest.DeviceConfigTest
Change-Id: I73cd0fae216ab9aecefc2fdb49a719e919c6437a