Implement OpenGL backend in Android viewer app

tools/viewer/sk_app/android/Window_android.h

 /*
 * Copyright 2016 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */
 
 #ifndef Window_android_DEFINED
 #define Window_android_DEFINED
 
+#include <android/native_window_jni.h>
+
 #include "../Window.h"
 #include "surface_glue_android.h"
 
 namespace sk_app {
 
+struct ContextPlatformData_android {
+    ANativeWindow* fNativeWindow;
+};
+
 class Window_android : public Window {
 public:
     Window_android() : Window() {}
     ~Window_android() override {}
 
     bool init(SkiaAndroidApp* skiaAndroidApp);
     void initDisplay(ANativeWindow* window);
     void onDisplayDestroyed();
 
     const DisplayParams& getDisplayParams() override;
     void setTitle(const char*) override;
     void show() override {}
 
     bool attach(BackendType attachType, const DisplayParams& params) override;
     void onInval() override;
     void setUIState(const Json::Value& state) override;
 
     void paintIfNeeded();
 
     bool scaleContentToFit() const override { return true; }
     bool supportsContentRect() const override { return true; }
     SkRect getContentRect() override { return fContentRect; }
     void setContentRect(int l, int t, int r, int b) { fContentRect.set(l,t,r,b); }
 
 private:
+    // We need fNativeWindow for attaching with another backend.
+    // (in that case, attach is called without initDisplay being called later)
+    ANativeWindow* fNativeWindow = nullptr;
     SkiaAndroidApp* fSkiaAndroidApp = nullptr;
     SkRect fContentRect;
     DisplayParams fDisplayParams;
+    BackendType fBackendType;
 };
 
 }   // namespace sk_app
 
 #endif