arc: Add support for ARC demo mode in isDemoUser

In ARC the system property ro.boot.arc_demo_mode is set in demo mode.
Updating the isDemoUser API to account for that so apps can behave as
expected.

Bug: 301629078
Test: atest UserManagerServiceDemoModeTest
Test: run ARC in demo mode with this change and ensure Play Store is in
demo mode

BYPASS_INCLUSIVE_LANGUAGE_REASON=blocked term is part of an API

Change-Id: I98c34e7138c6ccd958a49e239d53e5143e3d89b1