Store priority class as final in UnderlyingNetworkRecord

This commit refactors UnderlyingNetworkRecord to store
its priority class as a final field and calculate it
during construction. This commit improves the code
maintainabiliy in the network selection logic.

Bug: 245618320
Test: FrameworksVcnTests, CtsVcnTestCases
Change-Id: I346323cb18de935626b2dc6343be41c21fc7f055