Implement Network policy in VcnManagementService.

This CL implements #getUnderlyingNetworkPolicy() for
VcnManagementService. Specifically, it takes the provided
NetworkCapabilities and checks for a corresponding VCN. If one exists,
a VcnUnderlyingNetworkPolicy will be returned with a copy of the
original NetworkCapabilities without the capability NOT_VCN_MANAGED and
with the unwanted capability NOT_VCN_MANAGED.

Any NetworkCapabilities without a corresponding VCN will be returned a
VcnUnderlyingNetworkPolicy with the original NetworkCapabilities intact.

Bug: 175914059
Test: atest FrameworksVcnTests
Change-Id: Icdffc4f586f58a0e5b6290d5c6449e2bbed5cab0