Deprecate EXTRA_PROXY_INFO and unhide ConnectivityManager.getDefaultProxy().

1. Unhide ConnectivityManager.getDefaultProxy() and update it to
   take into account process-bound-Networks.
2. Deprecate EXTRA_PROXY_INFO and instead encourage querying via
   getDefaultProxy().

Bug: 17905627
Bug: 17420465
Bug: 18144582

Change-Id: I45358ee82fe705d048022c8238b2452f52c37b88

core/java/android/net/ConnectivityManager.java

@@ -1783,7 +1783,6 @@ public class ConnectivityManager {
      *
      * @return the {@link ProxyInfo} for the current HTTP proxy, or {@code null} if no
      *        HTTP proxy is active.
-     * @hide
      */
     public ProxyInfo getDefaultProxy() {
         final Network network = getBoundNetworkForProcess();