Set the specified update channel when silent auto update is enabled

This is an old policy for auto update. It is now recommended to use the new AU2 series policies. Default policy is 0, client will use 'Slow' as updating channel when the client auto updates silently. This policy depends on EnableSilentAutoUpdate, if EnableSilentAutoUpdate is disabled, SetUpdatingChannel will not work. If this policy is a valid integer, client will use the specified channel listed as below: 0 - Slow 1 - Fast If this policy is not set, client will use the default setting.