Use formal API for ActivityThread to set proxy

Add getProxyForNetwork to the public API, and use ConnectivityManager
APIs from ActivityThread (instead of hidden APIs) to get/set the proxy
for an app process.

getProxyForNetwork allows clients to find which proxy applies, which can
be a global proxy setting or a per-network proxy.

The default proxy is now initialized with getDefaultProxy instead of
getProxyForNetwork(null); this should not make a difference, as nothing
should have called bindProcessToNetwork at that point yet.

Bug: 174436414
Test: m; device boots
Change-Id: Ifb516194ecde1567cea4b6806946091cdcf2f015

framework/src/android/net/ConnectivityManager.java

@@ -4609,7 +4609,7 @@ public class ConnectivityManager {
             // Set HTTP proxy system properties to match network.
             // TODO: Deprecate this static method and replace it with a non-static version.
             try {
-                Proxy.setHttpProxySystemProperty(getInstance().getDefaultProxy());
+                Proxy.setHttpProxyConfiguration(getInstance().getDefaultProxy());
             } catch (SecurityException e) {
                 // The process doesn't have ACCESS_NETWORK_STATE, so we can't fetch the proxy.
                 Log.e(TAG, "Can't set proxy properties", e);