clang will silence availability errors in classes and methods that are also unavailable to the target. The following have been made unavailable to app extensions:
- `AFNetworkActivityIndicatorManager` class
- `UIAlertView` category methods
- `AFURLConnectionOperation setShouldExecuteAsBackgroundTaskWithExpirationHandler:`
Benefits of change:
- Fixes CocoaPods support for App Extensions.
- Allows single framework to be used in app.
- Eliminates the need for `AF_APP_EXTENSIONS`.