First prototype atttempting to support an external display

both API and implementation will change, this is just a prototype
intended to show feasability.

SurfaceFlinger is passed an ISurfaceTexture through a new
callback, it is in turn used to create an EGLSurface which
surfaceflinger will draw into in addition to the main screen.

Change-Id: Id0bbb0b854bb7bae44d57246a90b65d4567f9a21