Move Proxy, PacProxySelector out of Connectivity

PacProxySelector is tied to IProxyService, which does not have a formal
API.
Proxy is the interface with ConnectivityService, and all its methods are
public or module API.

Bug: 171540887
Test: m
Change-Id: I8ceba961a81661c3e11d8179955b594d3cab6ff7