Make VCN HSUM compatible

Require VCN to be used by a "main user" instead of a "system user"
since "system user" will no longer be a full user on HSUM devices

Bug: 310310661
Test: FrameworksVcnTests, CtsVcnTestCases on non-hsum and hsum devices
Flag: android.net.vcn.enforce_main_user
Change-Id: I5abfa7ba82055cd953083b1ab3f83e61bafd8a55