Support routing sockets as another user

Add support for routing sockets as if they were another user's.
This is for services that handle delegated network tasks like MediaServer and DownloadManager.

Change-Id: Id20efc1f5c2cce6f8838d777762f6c0a703a9437

core/java/android/net/IConnectivityManager.aidl

@@ -119,6 +119,8 @@ interface IConnectivityManager
 
     boolean prepareVpn(String oldPackage, String newPackage);
 
+    void markSocketAsUser(in ParcelFileDescriptor socket, int uid);
+
     ParcelFileDescriptor establishVpn(in VpnConfig config);
 
     void startLegacyVpn(in VpnProfile profile);