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