Update import class from TrafficStatsConstants to NetworkStackConstants

Some constants in TrafficStatsConstants are moved to
NetworkStackConstants so update the import class to use the constants.

Bug: 182349970
Test: FrameworksNetTests
      NetworkStackTests
      TetheringTests
Ignore-AOSP-First: this topic needs to be cherry-picked manually
to AOSP to avoid automerging differences.

Change-Id: Iaab5d721bb3b9536f560ad12436530150e03c370