Remove usage of Context.NETWORK_STACK_SERVICE

The NetworkStack.getService() API should be used instead.

Bug: 151243982
Test: atest FrameworksNetTests TetheringTests
      Manual tethering test
Merged-In: I7855090bffbe895c8349ad4903b8f2eb55515f0b
(clean cherry-pick from internal branch)

Change-Id: If4af2846a82605e828287a9a4680d5547b76b802