让ImageVIew按图片比率拉升至屏幕的width。

http://stackoverflow.com/questions/2991110/android-how-to-stretch-an-image-to-the-screen-width-while-maintaining-aspect-ra

http://stackoverflow.com/questions/4677269/how-to-stretch-three-images-across-the-screen-preserving-aspect-ratio

AspectRatioImageView