[ios-sdk] Fix FBViewController to put its canvas view behind any custom UI.
[ios-sdk] Fix FBViewController to put its canvas view behind any custom UI.

Summary:
canvasView was in front of UI being loaded from, e.g., a NIB, causing them not to receive UI events. Moved
it to the back so UI elements on top of it get events.

Test Plan:
- Ran Scrumptious, PlacePickerSample, (not committed) ViewControllersSample

Revert Plan:

Reviewers: jacl, mmarucheck, gregschechte, ayden

Reviewed By: jacl

CC: msdkexp@, platform-diffs@lists

Differential Revision: https://phabricator.fb.com/D526653

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