after an app is minimized, the app icon does not respond when you click
the app icon on the Desktop

solve problem: open some apps in one time, one app in it can not open later.
Because, task_id and id in change-475ca1c8u is not similar.

1.rename "register_event" -> "user_window_event",
  rename "get_register_event()" -> "get_user_window_event()".
2.add function user_event_function() to reduce Cyclomatic complexity of process_event
  in file platform/sdl/platform.cpp.
3.add Log when SDL_RegisterEvent return -1.
4.add lock in layer_composer.cpp:submit_layers to protect window in win_layers.

Set ime socket path from /tmp/ime to socket_dir()/ime_socket.
note: socket on android system is ime as old. So no need to change android code.

x11 impliments(X11_XSetICFocus, X11_XUnsetICFocus, X11_XDestroyIC, X11_XCreateIC) are "thread safe".
In anbox, they are called by SDL_WaitForEventTimeout, SDL_CreateWindow, SDL_DestroyWindow.
But origin anbox code sets them in different thread, and then crashed in a low probability.
This commit sets them in the same thread to avoid this crash.

Audio playback still has a delay about 1-2 seconds but it's usable.

This also moves the platform policy class into its own namespace to be
not specific to just window management.

* based on liblxc now
* container is managed by a management process which needs to run as
  root. LXC will take care of strict confinement of this container.
  We still don't allow any direct hardware access.
* `anbox shell` command is gone. Same functionality can be archive
  with the lxc-attach command.