Avoid hardcoding SYSTEM_USER to support HSUM

In case of headless system user mode, first human user would not be
SYSTEM_USER(user 0), so avoid harcoding for SYSTEM_USER.

Bug: 265281464
Test: atest android.devicepolicy.cts.ManagedSubscriptionsPolicyTest
Change-Id: I261867048a2a5b44d19ba5b34537b8d5201e4534