Use Context.NETWORK_POLICY_SERVICE to get network policy service

INetworkPolicyManager is a raw AIDL file which won't expose
as system API. Let's communicate with network policy service
through NetworkPolicyManager instead. Context.NETWORK_POLICY_SERVICE
is @SystemApi.

Bug: 138306002
Test: atest FrameworksNetTests
      atest FrameworksTelephonyTests

Change-Id: I41bc1cce08f81e2d822250f2d620bff7afe4329b
Merged-In: I41bc1cce08f81e2d822250f2d620bff7afe4329b