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
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:e8a2be08ea757e59c65abe52c8edcead8bb36038)

Change-Id: Ibecd45a68434834b3e2a1d35034f1986655c2910