Expose VCN Network policy APIs.

This CL exposes APIs used for a) receiving VCN Network policy updates
and b) applying VCN Network policies to a given Network. Specifically,
these APIs should be used to receive notifications from VcnManager
whenever a VCN Network policy may have changed - generally, following a
VCN starting up, tearing down, or entering Safe Mode.

When a VcnNetworkPolicyListener is notified of a policy change, the
listener should query VcnManager#applyVcnNetworkPolicy to receive the
updated VcnNetworkPolicyResult. This policy result informs the listener
of their resulting NetworkCapabilities and also whether a Network
teardown is required as a consequence of the policy change.

Bug: 177020190
Test: atest FrameworksVcnTests
Change-Id: Ied78a57973a4ceaa7ef4cd9feee64478137497b8