Returning a confirmation of the cancel request.
Returning a confirmation of the cancel request.

Summary: If callers to ImageDownloader are doing any sort of bookkeeping of requests they issue, it would be helpful to know when calls to cancelRequest are successful and when not.

Test Plan: Ran ProfilePictureSample which is the only place that exercises cancel right now

Reviewers: clang, mmarucheck, mingfli

Reviewed By: clang

CC: msdkexp@

Differential Revision: https://phabricator.fb.com/D612553

Task ID: 1771141

MarkHappy/Facebook_SDK,bitcellar-labs/facebook-android-sdk,TechSmith/facebook-android-sdk,RabbleApp/facebook-android-sdk,GrioSF/facebook-android-sdk,Limbika/facebook-android-sdk,satoshi-okawa/facebook-android-sdk-test1,couchbaselabs/facebook-android-sdk,MKrishtop/facebook-android-sdk,Flutterbee/facebook-android-sdk,MKrishtop/facebook-android-sdk,quizlet/facebook-android-sdk,lizhi5753186/facebook-android-sdk,TechSmith/facebook-android-sdk,GrioSF/facebook-android-sdk,TimeIncOSS/facebook-android-sdk,couchbaselabs/facebook-android-sdk,taisukeoe/facebook-android-sdk,mindjolt/facebook-android-sdk,bitcellar-labs/facebook-android-sdk,scalio/facebook-android-sdk,minhduc3404/facebookSdk,quizlet/facebook-android-sdk,TimeIncOSS/facebook-android-sdk,PioneerLab/facebook-android-sdk,RabbleApp/facebook-android-sdk,scalio/facebook-android-sdk,minhduc3404/facebookSdk,Flutterbee/facebook-android-sdk,PioneerLab/facebook-android-sdk,ribot/facebook-android-sdk,ribot/facebook-android-sdk,Limbika/facebook-android-sdk,CristianOliveiraDaRosa/facebook-android-sdk,CristianOliveiraDaRosa/facebook-android-sdk,MarkHappy/Facebook_SDK,satoshi-okawa/facebook-android-sdk-test1,lizhi5753186/facebook-android-sdk,taisukeoe/facebook-android-sdk,mindjolt/facebook-android-sdk