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

framework/api/module-lib-current.txt

@@ -23,10 +23,6 @@ package android.net {
     field public static final int TRANSPORT_TEST = 7; // 0x7
   }
 
-  public final class Proxy {
-    method public static void setHttpProxyConfiguration(@Nullable android.net.ProxyInfo);
-  }
-
   public final class TcpRepairWindow {
     ctor public TcpRepairWindow(int, int, int, int, int, int);
     field public final int maxWindow;