Add VcnUtils to easily get WifiInfo and subId from NetworkCapabilities

This is a preparation CL for mainlining VCN. This patch adds utility
methods for callers to easily get WifiInfo and subId. It also adds
a Builder class that will be exposed as a @SystemApi in a followup
CL.

Bug: 369710077
Test: atest FrameworksVcnTests(new tests) && atest CtsVcnTestCases
Flag: Exempt low risk utilities for pure refactoring
Change-Id: I257b031dabfc48286e8bae2a0854c81aca2f189c