Split out NetworkPriorityClassifier and UnderlyingNetworkRecord

Move UnderlyingNetworkRecord to a spearate file, and
move calculatePriorityClass to a new class NetworkPriorityClassifier.

This is a preparation CL to support vcn routeselection with
configurable network priority classes and network metric
monitors.

Bug: 206044122
Test: atest FrameworksVcnTests, CtsVcnTestCases
Change-Id: I88aa9e8dcf01524ddce9f913f40a4c712f8c878d