默认允许安装未知来源的应用

 frameworks/base/core/java/android/app/AppOpsManager.java   
/**
     * This specifies the default mode for each operation.
     */
    private static int[] sOpDefaultMode = new int[] {
            AppOpsManager.MODE_ALLOWED, // COARSE_LOCATION
            AppOpsManager.MODE_ALLOWED, // FINE_LOCATION
			...
			AppOpsManager.MODE_ALLOWED, // REQUEST_INSTALL_PACKAGES----modify this line
			...