Provide a sample of multi-display wallpaper.

Some key events to build a wallpaper which can support
multiple display.
- The rendering or event handler of each engine must be
independent.
- Set supportsMultipleDisplays attribute to true.
- Use Engine#getDisplayContext to load resource if
needed.

Bug: 129696884
Test: launch wallpaper and check on multi display.
Change-Id: I3c60caba921e0af3d5f681ed318a543328e3375a