Move fromPublicNetworkStats to statsd

This is a no-op refactor to reduce complexity to prevent from
accessing trunk stable flags from static libs. Also it
makes more sense since statsd is the only caller.

Test: atest CtsStatsdAtomHostTestCases:android.cts.statsdatom.net.BytesTransferredTest
Test: adb shell cmd stats pull-source 10000
Test: atest FrameworksServicesTests:com.android.server.stats.pull.netstats.NetworkStatsUtilsTest
Bug: 335680025
Change-Id: I1674fcb06df7c90d063f8dc9b2644b465be09feb