[AF01] Move NetworkStatsCollection comparison code to NetworkStatsCollection

This is a no-op refactoring to move the ultility method to a
common place to be referenced outside NetworkStatsService
in later patches.

Test: atest ConnectivityCoverageTests:android.net.connectivity.com.android.server.net.NetworkStatsServiceTest
Bug: 313841499
Change-Id: Ib0ef40d87486bf3d8d5b3f97c77291a00c0378ac