Enable client to auto update silently

This is an old policy for auto update. It is now recommended to use the new AU2 series policies. Default policy is enabled. This policy depends on EnableClientAutoUpdate, if EnableClientAutoUpdate is disabled, EnableSilentAutoUpdate will not work, even if it is enabled. (For msi package, EnableClientAutoUpdate is disabled by default. For exe, it is enabled) If this policy is enabled, client will auto update itself silently. If this policy is disabled, client will auto update itself and prompt users the update dialog. If this policy is not set, client will use the default setting.