NetworkPolicyManager: Add @SystemApi for mainlne module

1) Add NetworkPolicyManager.registerSubscriptionCallback and
   NetworkPolicyManager.unregisterSubscriptionCallback for
   registering and unregister.
2) Create SubscriptionCallback to support new @SystemApi
   so that caller could use this object to register and
   unregister.

Bug: 138306002
Test: FrameworksNetTests
      FrameworksTelephonyTests

Change-Id: I56833254a93383e8054c96d296bcb54b777a6e33