Don't use isSystemUser for UserItemsCombiner

For devices which don't have an actual user as the system user, this
re-orders the user items incorrectly. Check the managed profile instead
of using the isSystemUser.

Bug: 408066117
Test: atest com.android.documentsui.sidebar.UserItemsCombinerTest
Flag: EXEMPT but fix
Change-Id: Iab80ccad4f7ba5218a23eccb38e75e2834f22bae