Android 4: 設置背景為系統wallpaper

1. <style name="Theme.Holo.Wallpaper">  
2.     <item name="android:windowBackground">@android:color/transparent</item>  
3.     <item name="android:colorBackgroundCacheHint">@null</item>  
4.     <item name="android:windowShowWallpaper">true</item>  
5. </style>  

直接用Theme_Holo_Wallpaper或者在自定義的Theme中增加這3個item