Split BpfNetMaps constants and utilities into standalone classes

This is a no-op refactoring that splits constants and utility
methods into standalone classes which will be shared with
a bpf reader class in subsequent CLs.

NO_IFTTT=No-op refactoring

Test: atest FrameworksNetTests:android.net.connectivity.com.android.server.BpfNetMapsTest
Test: atest ConnectivityCoverageTests:android.net.connectivity.com.android.net.module.util.StructTest

Bug: 297836825
Change-Id: I6d7ea044e43180ae001573009a166be74ebe6a5d