Simplify the VPN service implementation.

+ Remove NormalProcessProxy and ProcessProxy as they are not used
  anymore.
+ Rename AndroidServiceProxy to DaemonProxy and simplify its
  implementation as it does not extend to ProcessProxy anymore.
+ Execute connect() in VpnService in one thread, which simplifies socket
  and error handling.
+ Modify service subclasses accordingly.
+ Execute connect() and disconnect() in VpnServiceBinder so that the
  operations do not block the UI thread. Mark service as foreground only upon
  connecting.