Refactor rewards service GetPatamaters method

### Overview
Refactor the Rewards service [GetParamaters](https://github.com/brave-intl/bat-go/blob/3d4841534d28c1a35b9f9c99e5af1378f800c212/services/rewards/service.go#L88) method.

#### Tasks
* Remove dependencies on ctx for config, for example, ctx.Value(appctx.ParametersVBATDeadlineCTXKey).(time.Time)
* Improve testing and testability where possible