Add CallSuper annotation to onCreate(). (#4789)

This provides improved code inspection, making it easier for developers to figure out what might be going wrong if they override this and don't call super, like I just did. :-)

shell/platform/android/io/flutter/app/FlutterApplication.java

@@ -6,6 +6,7 @@ package io.flutter.app;
 
 import android.app.Activity;
 import android.app.Application;
+import android.support.annotation.CallSuper;
 
 import io.flutter.view.FlutterMain;
 
@@ -15,6 +16,7 @@ import io.flutter.view.FlutterMain;
  */
 public class FlutterApplication extends Application {
     @Override
+    @CallSuper
     public void onCreate() {
         super.onCreate();
         FlutterMain.startInitialization(this);