Use FakePermissionEnforcer for NetworkManagementServiceTest

Update the test to use the new FakePermissionEnforcer class which
contains the logic to verify if a permission has been artificially
granted.

Bug: 269684922
Test: atest FrameworksServicesTests:com.android.server.net.NetworkManagementServiceTest
Change-Id: Iea4ec02d4be27f8043c1a57db11cadfd6d90b3ba