Fix android VideoPlayer size calculation when keepRatio == true.

Cocos2dxVideoView has problem when `videoPlayer->setKeepAspectRatioEnabled(true);`

# reproduce step
## modify code
1. Please Go `UIVideoPlayerTest.cpp` -> `VideoPlayerTest::createVideo()`
2. modify code same as ↓this.

This code want to show video that fit screen.

```cpp
void VideoPlayerTest::createVideo()
{
    auto centerPos = Vec2(_visibleRect.origin.x + _visibleRect.size.width / 2,_visibleRect.origin.y + _visibleRect.size.height /2);

    auto widgetSize = _widget->getContentSize();

    _videoPlayer = VideoPlayer::create();
    _videoPlayer->setPosition(centerPos);
    _videoPlayer->setAnchorPoint(Vec2::ANCHOR_MIDDLE);
    // I modify this line.
    _videoPlayer->setContentSize(Director::getInstance()->getVisibleSize());
    _videoPlayer->setKeepAspectRatioEnabled(true);

    _uiLayer->addChild(_videoPlayer);
    
    _videoPlayer->addEventListener(CC_CALLBACK_2(VideoPlayerTest::videoEventCallback, this));
}
 _videoPlayer->addEventListener(CC_CALLBACK_2(VideoPlayerTest::videoEventCallback, this));
```

## run android cpp-tests
1. Run `cpp-tests`.
2. Go to `Node::UI -> GUI Dynamic Create Test -> VideoPlayerTest`
3. Tap `Play resource video` label
4. problem reproduce
5. Tap `KeepRationSwitch`
6. Another problem reproduce

I submit another pr for fix this issue at https://github.com/cocos2d/cocos2d-x/pull/7597