Implement OpenGL backend in Android viewer app

tools/viewer/sk_app/android/GLWindowContext_android.h

Index: tools/viewer/sk_app/android/GLWindowContext_android.h
diff --git a/tools/viewer/sk_app/unix/GLWindowContext_unix.h b/tools/viewer/sk_app/android/GLWindowContext_android.h
similarity index 56%
copy from tools/viewer/sk_app/unix/GLWindowContext_unix.h
copy to tools/viewer/sk_app/android/GLWindowContext_android.h
index 2d76254c21012d710557dae0c2f00aff56452305..85e65beaa251b451cb41ab3d98d0cc33f178496c 100644
--- a/tools/viewer/sk_app/unix/GLWindowContext_unix.h
+++ b/tools/viewer/sk_app/android/GLWindowContext_android.h
@@ -5,19 +5,21 @@
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
-#ifndef GLWindowContext_unix_DEFINED
-#define GLWindowContext_unix_DEFINED
+#ifndef GLWindowContext_android_DEFINED
+#define GLWindowContext_android_DEFINED
 
 #include "../GLWindowContext.h"
-#include "Window_unix.h"
+#include "Window_android.h"
+
+#include <EGL/egl.h>
 
 namespace sk_app {
 
-class GLWindowContext_unix : public GLWindowContext {
+class GLWindowContext_android : public GLWindowContext {
 public:
     friend GLWindowContext* GLWindowContext::Create(void* platformData, const DisplayParams&);
 
-    ~GLWindowContext_unix() override;
+    ~GLWindowContext_android() override;
 
     void onSwapBuffers() override;
 
@@ -25,12 +27,11 @@ public:
     void onDestroyContext() override;
 
 private:
-    GLWindowContext_unix(void*, const DisplayParams&);
+    GLWindowContext_android(void*, const DisplayParams&);
 
-    Display*     fDisplay;
-    XWindow      fWindow;
-    XVisualInfo* fVisualInfo;
-    GLXContext   fGLContext;
+    EGLDisplay fDisplay;
+    EGLContext fEGLContext;
+    EGLSurface fSurface;
 };