Replace INetworkPolicyManager to NetworkPolicyManager

Connectivity service is going to become a mainline module which
will not able to access hidden APIs. Thus, use formal API
Context#getSystemService() to get network policy service instead
of hidden API ServiceManager#getService().

Bug: 170598012
Test: atest FrameworksNetTests FrameworksNetIntegrationTests
Change-Id: I4f286264b5800b2b922f85a76ddd20d64d53000a