[android-sdk] Remove redundant activity definition from SessionLoginSample manifest
[android-sdk] Remove redundant activity definition from SessionLoginSample manifest

Summary:
.MainActivity was declared twice in AndroidManifest.xml. On some devices this caused an exception on startup.
Removed the redundant definition.

Test Plan:
- Ran SessionLoginSample

Revert Plan:

Reviewers: mmarucheck, mingfli, karthiks

Reviewed By: mingfli

Differential Revision: https://phabricator.fb.com/D635289

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