Define AF_APP_EXTENSION based on availability with the preprocessor

As seen on Availability.h
Merge pull request #2346 from dtorres/auto-extension-availability

Define AF_APP_EXTENSION based on availability with the preprocessor
[Issue #2346] Refactoring preprocessor directives to remove reference to AF_APP_EXTENSION
[Issue #2346] Adding preprocessor directives around UIApplication network activity indicator
[Issue #2346] Fix preprocessor directive to pass if __has_feature is unavailable
[Issue #2346] Conditionally compile AFNetworkActivityManagerTests
[Issue #2346] Revert commits related to __has_feature(attribute_availability_app_extension)