Use swipe gesture to switch between slides on Android

tools/viewer/sk_app/android/surface_glue_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 surface_glue_android_DEFINED
 #define surface_glue_android_DEFINED
 
 #include <pthread.h>
 
 #include <android/native_window_jni.h>
 
 #include "../Application.h"
 #include "../Window.h"
 
 namespace sk_app {
 
 enum MessageType {
     kUndefined,
     kSurfaceCreated,
     kSurfaceChanged,
     kSurfaceDestroyed,
     kDestroyApp,
-    kContentInvalidated
+    kContentInvalidated,
+    kKeyPressed
 };
 
 struct Message {
     MessageType fType = kUndefined;
     ANativeWindow* fNativeWindow = nullptr;
+    int keycode = 0;
 
     Message() {}
     Message(MessageType t) : fType(t) {}
 };
 
 struct SkiaAndroidApp {
     int fPipes[2];  // 0 is the read message pipe, 1 is the write message pipe
     Application* fApp;
     Window* fWindow;
     ANativeWindow* fNativeWindow;
 
     SkiaAndroidApp();
     ~SkiaAndroidApp();
     void postMessage(const Message& message);
     void readMessage(Message* message);
     void paintIfNeeded();
 
 private:
     pthread_t fThread;
 };
 
 }  // namespace sk_app
 
 #endif