Set the specified update channel when auto-update feature is enabled (Recommendation Setting)

Default policy is empty, client will subscribe 'Slow' as update channel when the auto-update feature is enabled. This policy depends on AU2_EnableAutoUpdate, if AU2_EnableAutoUpdate is disabled, AU2_SetUpdateChannel will not work. If this policy is a valid string, 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.