Migrate NetworkStack metrics off StatsLog.write

Now that the new metrics API can be used, NetworkStack
metrics need to migrate off the legacy StatsLog.write.

Bug: 130323000
Test: Test with ./out/host/linux-x86/bin/statsd_testdrive 121
      to ensure log properly
Test: Ensure NetworkStackStatsLog will be generated as expected
Test: atest NetworkStackTest

Change-Id: I8c70503cf1d9f1d06f30a1936d2d88857d0cff53