Add ApiDemos for secure surfaces.

Added three ApiDemos for secure surfaces.  One for Activity,
one for Dialog and one for SurfaceView.  The mechanism is the
same in each case but the API is a little different.

Bug: 7368436
Change-Id: I78bc2a456b4c0f1a553120d72f433095ba7e038c

samples/ApiDemos/src/com/example/android/apis/graphics/CubeRenderer.java

@@ -25,7 +25,7 @@ import android.opengl.GLSurfaceView;
  * Render a pair of tumbling cubes.
  */
 
-class CubeRenderer implements GLSurfaceView.Renderer {
+public class CubeRenderer implements GLSurfaceView.Renderer {
     public CubeRenderer(boolean useTranslucentBackground) {
         mTranslucentBackground = useTranslucentBackground;
         mCube = new Cube();