Allow MANAGE_TEST_NETWORKS to register and get VCN policies.

This CL updates VcnManagementService to allow permission
MANAGE_TEST_NETWORKS to register/unregister VCN policy listers as
well as apply VCN policies. Previously, only permission
NETWORK_FACTORY was allowed to perform this operations.

Bug: 189125789
Test: atest FrameworksVcnTests CtsVcnTestCases
Change-Id: I6ad3a58f4ef87d931917fbd772a810af81b27da1