Adjust permission of NetworkProvider related API

- Allow an app holds NETWORK_SETTINGS to acess registerNetworkProvier()
  and unregisterNetworkProvider().
- To access declareNetworkRequestUnfulfillable(), allow an app holds
  MANAGE_TEST_NETWORKS to declare a unfulfillable request that contains
  TRANSPORT_TEST transport.

This makes easier to write cts to test.

Bug: 153612373
Bug: 153614605
Test: atest FrameworksNetTests
      atest CtsNetTestCases:android.net.NetworkProviderTest
Change-Id: Ic9809e731aa811a51c2f82d189372169d99a5ed9