Change `firstPingSuccess` Behavior

[jhreis]

https://github.com/brave/brave-ios/pull/399/files#diff-4cd4e2c0264423573c81795757e55822R88

We have some code like:

/// First ping preference is set after first successful ping to the server.
let firstPing = Preferences.DAU.firstPingSuccess.value
// First launch ping should be send only if first ping was not successful yet.
let firstLaunch = !firstPing

We should modify firstPingSuccess to be more clear in its purpose. This is new for 1.7, so modifying before launch would be best.

Could even name it something like firstPingRequired, to modify the logic, as right now inverting the result let firstLaunch = !firstPing is a bit odd.