Cleanup when destroying view and engine (#304)

* The view should be responsible for the life cycle of the engine, and
  the engine should stop naturally when the view is destroyed.
* There is no need to explicitly assign null to a unique pointer in
  the destructor of engine.

Signed-off-by: Boram Bae <boram21.bae@samsung.com>

shell/platform/tizen/flutter_tizen_engine.cc

@@ -91,7 +91,7 @@ FlutterTizenEngine::FlutterTizenEngine(const FlutterProjectBundle& project)
 }
 
 FlutterTizenEngine::~FlutterTizenEngine() {
-  renderer_ = nullptr;
+  StopEngine();
 }
 
 bool FlutterTizenEngine::RunEngine() {

shell/platform/tizen/flutter_tizen_view.cc

@@ -53,7 +53,9 @@ FlutterTizenView::FlutterTizenView(std::unique_ptr<TizenViewBase> tizen_view)
   }
 }
 
-FlutterTizenView::~FlutterTizenView() {}
+FlutterTizenView::~FlutterTizenView() {
+  DestroyRenderSurface();
+}
 
 void FlutterTizenView::SetEngine(std::unique_ptr<FlutterTizenEngine> engine) {
   engine_ = std::move(engine);