App Hange Rate Apple: Add feature flag

[philipphofmann]

Most Apple projects won't receive any data to calculate a correct app hang rate, as users need to enable an experimental feature in the Cocoa SDK. Therefore, it makes sense to add feature flag for the app hangs rate and only enable it per default once the experimental feature in the Cocoa SDK is stable. Furthermore, it allows us to open smaller PRs without impacting the project dashboard for existing users.