Define VcnNetworkPolicyListener for VCN transports.

This CL defines VcnNetworkPolicyListener and VCN Network policy
functions for VcnManager. Specifically, VcnNetworkPolicyListeners should
be added and removed by Network factories as they bring up and teardown
data Networks.

VcnNetworkPolicyListeners are used by VcnManagementService to notify
transports when they should re-query for their updated
VcnNetworkPolicyResults - done via VcnManager#applyVcnNetworkPolicy.
Policy results inform transports what the current NetworkCapabilities
are based on their VCN policy, and also whether the transport should
teardown and restart.

Bug: 177020190
Test: atest FrameworksVcnTests
Change-Id: Ide2c73025f4ffceabf7955f219a79be5a9bdc0d9