SOLID: Dependency Inversion Principle

While I was working on refactoring the Notification Settings of our app (from #Swift UIKit to #SwiftUI), I applied this principle because it made me think of the old Push Notification service we used before, which was Firebase (and now, OneSignal).

With Dependency Inversion Principle, we can change the service any time without worrying if we will break other parts of the code.