BraveAdsRedeemRewardConfirmationStudy

Reduce the delay for fetching the payment token after redeeming confirmation tokens from 15 seconds to 3 seconds. During a call with @tackley, we reviewed server response times and determined that 3 seconds was the optimal value. Related browser change https://github.com/brave/brave-core/pull/26641 to ride the trains. This feature has been live for over 7 months.