[ios-sdk] Don't want userAgent static to autorelease.
[ios-sdk] Don't want userAgent static to autorelease.

Summary:
Earlier change mistakenly removed a retain call from the userAgent string, which meant
it was getting autoreleased, with predictable results. We don't ever want to release it.

Test Plan: - Ran unit tests, didn't crash

Reviewers: jacl, mmarucheck, gregschechte

Reviewed By: jacl

Differential Revision: https://phabricator.fb.com/D489225

Citrrus/facebook-ios-sdk,ShareKit/facebook-ios-sdk,flovilmart/facebook-ios-sdk,FiftyThree/facebook-ios-sdk,wooga/facebook-ios-sdk,anthonylai/facebook-ios-sdk,taptaptap/facebook-ios-sdk,1234-/facebook-ios-sdk,wooga/facebook-ios-sdk,flovilmart/facebook-ios-sdk,taptaptap/facebook-ios-sdk,ShareKit/facebook-ios-sdk,shepting/facebook-ios-sdk,shepting/facebook-ios-sdk,shepting/facebook-ios-sdk,beamly/facebook-ios-sdk,Citrrus/facebook-ios-sdk,tipbit/facebook-ios-sdk,TimeIncOSS/facebook-ios-sdk,ShareKit/facebook-ios-sdk,shepting/facebook-ios-sdk,Amnysia/facebook-ios-sdk,hibu/facebook-ios-sdk,flovilmart/facebook-ios-sdk,Subsplash/facebook-ios-sdk,Amnysia/facebook-ios-sdk,FiftyThree/facebook-ios-sdk,FiftyThree/facebook-ios-sdk,Subsplash/facebook-ios-sdk,ArtLapitski/facebook-ios-sdk,1234-/facebook-ios-sdk,Subsplash/facebook-ios-sdk,hibu/facebook-ios-sdk,TimeIncOSS/facebook-ios-sdk,flovilmart/facebook-ios-sdk,1234-/facebook-ios-sdk,shepting/facebook-ios-sdk,tipbit/facebook-ios-sdk,flovilmart/facebook-ios-sdk,beamly/facebook-ios-sdk,wooga/facebook-ios-sdk,taptaptap/facebook-ios-sdk,anthonylai/facebook-ios-sdk,hibu/facebook-ios-sdk,Citrrus/facebook-ios-sdk,TimeIncOSS/facebook-ios-sdk,anthonylai/facebook-ios-sdk,ArtLapitski/facebook-ios-sdk,beamly/facebook-ios-sdk,ArtLapitski/facebook-ios-sdk,tipbit/facebook-ios-sdk,1234-/facebook-ios-sdk,hibu/facebook-ios-sdk,Amnysia/facebook-ios-sdk,1234-/facebook-ios-sdk,hibu/facebook-ios-sdk,ArtLapitski/facebook-ios-sdk