Implement Policy Listener add/remove in VcnService.

This CL updates VcnManagementService to implement
VcnUnderlyingNetworkPolicyListener add and remove operations. It also
adds functionality to remove any listeners if the registering app dies
without unregistering.

Bug: 175739863
Test: atest FrameworksVcnTests
Change-Id: I9db2729d404f232b84577d2d90909b9152d53b5a