Set POLICY_VCN in VCN network offer to fix safe mode

This patch sets POLICY_VCN in its network offer. This new policy
allows VCN to win over a physical cell network and thus fixs the issue
where VCN cannot exit safe mode when there is already a phsical cell

Bug: 391977557
Test: CtsVcnTestCases(new test)
Flag: EXEMPT bugfix
Change-Id: I68bea55c96051d0c7510df3713bfc52ff3460a01