Update Policy Listener API naming.

This CL changes the policy listener API to be
VcnNetworkPolicyChangeListener (it was previously
VcnNetworkPolicyListener) per API Council guidance.

This CL also requires permission NETWORK_FACTORY for removing registered
policy listeners.

Bug: 181562364
Test: atest FrameworksVcnTests
Change-Id: I026eaefa62d8f64b9180fc182a7cf0605d83bf97